Shia ( /ˈʃiːə/ ; Arabic : شيعة Shīʿah , from Shīʻatu ʻAlī , "followers of Ali") is a branch of Islam which holds that the Islamic prophet Muhammad designated Ali ibn Abi Talib as his successor ( Imam ).  Shia Islam primarily contrasts with Sunni Islam , whose adherents believe that Muhammad did not appoint a successor and consider Abu Bakr (who was appointed Caliph through a Shura , i.e. community consensus) to be the correct Caliph.  Unlike the first three Rashidun caliphs, Ali was from the same clan as Muhammad, Banu Hashim .
Adherents of Shia Islam are called Shias of Ali , Shias or the Shi'a as a collective or Shi'i individually. Shia Islam is the second-largest branch of Islam: in 2009, Shia Muslims constituted 10–13% of the world's Muslim population.  Twelver Shia ( Ithnā'ashariyyah ) is the largest branch of Shia Islam.  In 2012 it was estimated that perhaps 85 percent of Shias were Twelvers. 
Shia Islam is based on the Quran and the message of Muhammad attested in hadith , and on hadith taught by their Imams .  Shia consider Ali to have been divinely appointed as the successor to Muhammad, and as the first Imam. The Shia also extend this Imammah doctrine to Muhammad's family, the Ahl al-Bayt ("the people/family of the House"  ), and some individuals among his descendants, known as Imams , who they believe possess special spiritual and political authority over the community, infallibility and other divinely ordained traits.  Although there are many Shia subsects , modern Shia Islam has been divided into three main groupings: Twelvers , Ismailis and Zaidis , with Twelver Shia being the largest and most influential group among Shia.   
The word Shia ( Arabic : شيعة shīʻah /ˈʃiːʕa/ ) means follower  and is the short form of the historic phrase shīʻatu ʻAlī ( شيعة علي /ˈʃiːʕatu ˈʕaliː/ ), meaning "followers of Ali", "faction of Ali", or "party of Ali". Shi'a and Shiism are forms used in English, while Shi'ite or Shiite , as well as Shia , refer to its adherents.
The term for the first time was used at the time of Muhammad.  At present, the word refers to the Muslims who believe that the leadership of the community after Muhammad belongs to Ali and his successors. Nawbakhti states that the term Shia refers to a group of Muslims that at the time of Muhammad and after him regarded Ali as the Imam and Caliph.  Al-Shahrastani expresses that the term Shia refers to those who believe that Ali is designated as the Heir, Imam and caliph by Muhammad  and also Ali's authority never goes out of his descendants.  For the Shia, this conviction is implicit in the Quran and history of Islam. Shia scholars emphasize that the notion of authority is linked to the family of the prophets as the verses 3:33,34 shows: "Indeed, Allah chose Adam and Noah and the family of Abraham and the family of 'Imran over the worlds - (33) Descendants, some of them from others. And Allah is Hearing and Knowing. (34)"  Shia search for the true meaning of the revelation to get the purpose of the life blood and the human destiny. 
Succession of Ali
Shia Muslims believe that just as a prophet is appointed by God alone, only God has the prerogative to appoint the successor to his prophet. They believe God chose Ali to be Muhammad's successor, infallible, the first caliph ( khalifa , head of state) of Islam. The Shias believe that Muhammad designated Ali as his successor by God's command.  
Ali was Muhammad's first cousin and closest living male relative as well as his son-in-law, having married Muhammad's daughter Fatimah . Ali would eventually become the fourth Muslim (Sunni) caliph.
After the Farewell Pilgrimage , Muhammad ordered the gathering of Muslims at the pond of Khumm and it was there that Shia Muslims believe Muhammad nominated Ali to be his successor. The hadith of the pond of Khumm was narrated on 18th of Dhu al-Hijjah of 10 AH in the Islamic calendar (10 March 632 AD) at a place called Ghadir Khumm , located near the city of al-Juhfah, Saudi Arabia.  Muhammad there stated:
Oh people! Reflect on the Quran and comprehend its verses. Look into its clear verses and do not follow its ambiguous parts, for by Allah, none shall be able to explain to you its warnings and its mysteries, nor shall anyone clarify its interpretation, other than the one that I have grasped his hand, brought up beside myself, [and lifted his arm,] the one about whom I inform you that whomever I am his master ( Mawla ), then Ali is his master ( Mawla ); and he is Ali Ibn Abi Talib, my brother, the executor of my will ( Wasiyyi ), whose appointment as your guardian and leader has been sent down to me from Allah, the mighty and the majestic.— Muhammad, from The Farewell Sermon
- The word mawla has many meanings as discussed in the book "Patronate And Patronage in Early And Classical Islam" By Monique Bernards, John Nawas on page 25:
" [M]awla may refer to a client, a patron, an agnate (brother, son, father's brother, father' brothers son), an affined kinsman, (brother-in-law, son-in-law), a friend, a supporter, a follower, a drinking companion, a partner, a newly-converted Muslim attached to a Muslim and last but not least an ally. Most of these categories have legal implications. In Islamic times, the term mawala mostly referred to Muslim freedmen and freed non-Arabs who attached themselves to Arabs upon their conversion to Islam. In these senses, Mawla is commonly translated as "a client". The association of mawla with non-Arabs and a low status imparted an increasingly pejorative connotation to it. 
Shia Muslims believe this to be Muhammad's appointment of Ali as his successor.
When Muhammad died in 632 CE, Ali and Muhammad's closest relatives made the funeral arrangements. While they were preparing his body, Abu Bakr , Umar , and Abu Ubaidah ibn al Jarrah met with the leaders of Medina and elected Abu Bakr as caliph. Initially, Ali did not accept the caliphate of Abu Bakr and refused to pledge allegiance to him. This is indicated in both Sunni and Shia sahih and authentic Hadith. 
I am the servant of God and the brother of the Messenger of God. I am thus more worthy of this office than you. I shall not give allegiance to you [Abu Bakr & Umar] when it is more proper for you to give bay’ah to me. You have seized this office from the Ansar using your tribal relationship to the Prophet as an argument against them. Would you then seize this office from us, the ahl al-bayt by force? Did you not claim before the Ansar that you were more worthy than they of the caliphate because Muhammad came from among you (but Muhammad was never from AbuBakr family) – and thus they gave you leadership and surrendered command? I now contend against you with the same argument…It is we who are more worthy of the Messenger of God, living or dead. Give us our due right if you truly have faith in God, or else bear the charge of wilfully doing wrong... Umar, I will not yield to your commands: I shall not pledge loyalty to him.' Ultimately Abu Bakr said, "O 'Ali! If you do not desire to give your bay'ah, I am not going to force you for the same. 
Ali 's wife, and daughter of Muhammad , Fatimah , refused to pledge allegiance to Abu Bakr and remained angry with him until she died due to the issues of Fadak and her inheritance from her father and the situation of Umar at Fatimah's house . This is stated in sahih Sunni Hadith, Sahih Bukhari and Sahih Muslim. Fatimah did not at all pledge allegiance or acknowledge or accept the caliphate of Abu Bakr .  Almost all of Banu Hashim , Muhammad 's clan and many of the sahaba , had supported Ali 's cause after the demise of the prophet whilst others supported Abu Bakr .    
It was not until the murder of the third caliph, Uthman , in 657 CE that the Muslims in Medina in desperation invited Ali to become the fourth caliph as the last source, and he established his capital in Kufah in present-day Iraq.
Ali's rule over the early Muslim community was often contested, and wars were waged against him. As a result, he had to struggle to maintain his power against the groups who betrayed him after giving allegiance to his succession, or those who wished to take his position. This dispute eventually led to the First Fitna , which was the first major civil war within the Islamic Caliphate. The Fitna began as a series of revolts fought against Ali ibn Abi Talib, caused by the assassination of his political predecessor, Uthman ibn Affan. While the rebels who accused Uthman of prejudice affirmed Ali's khilafa (caliph-hood), they later turned against him and fought him. Ali ruled from 656 CE to 661 CE, when he was assassinated while prostrating in prayer ( sujud ). Ali's main rival Muawiyah then claimed the caliphate.
Hasan ibn Ali
Upon the death of Ali, his elder son Hasan became leader of the Muslims of Kufa, and after a series of skirmishes between the Kufa Muslims and the army of Muawiyah, Hasan agreed to cede the caliphate to Muawiyah and maintain peace among Muslims upon certain conditions: 
- The enforced public cursing of Ali, e.g. during prayers, should be abandoned
- Muawiyah should not use tax money for his own private needs
- There should be peace, and followers of Hasan should be given security and their rights
- Muawiyah will never adopt the title of Amir al-Mu'minin
- Muawiyah will not nominate any successor
Hasan then retired to Medina, where in 670 CE he was poisoned by his wife Ja'da bint al-Ash'ath ibn Qays, after being secretly contacted by Muawiyah who wished to pass the caliphate to his own son Yazid and saw Hasan as an obstacle.
Husayn, Ali's younger son and brother to Hasan, initially resisted calls to lead the Muslims against Muawiyah and reclaim the caliphate. In 680 CE, Muawiyah died and passed the caliphate to his son Yazid , and breaking the treaty with Hasan ibn Ali. Yazid asked Husayn to swear allegiance ( bay'ah ) to him. Ali's faction, having expected the caliphate to return to Ali's line upon Muawiyah's death, saw this as a betrayal of the peace treaty and so Husayn rejected this request for allegiance. There was a groundswell of support in Kufa for Husayn to return there and take his position as caliph and imam, so Husayn collected his family and followers in Medina and set off for Kufa. En route to Kufa, he was blocked by an army of Yazid's men (which included people from Kufa) near Karbala (modern Iraq), and Husayn and approximately 72 of his family and followers were killed in the Battle of Karbala .
The Shias regard Husayn as a martyr ( shahid ), and count him as an Imam from the Ahl al-Bayt. They view Husayn as the defender of Islam from annihilation at the hands of Yazid I . Husayn is the last imam following Ali whom all Shiah sub-branches mutually recognize. The Battle of Karbala is often cited as the definitive break between the Shiah and Sunni sects of Islam, and is commemorated each year by Shiah Muslims on the Day of Ashura .
Imamate of the Ahl al-Bayt
Most of the early Shia differed only marginally from mainstream Sunnis in their views on political leadership, but it is possible in this sect to see a refinement of Shia doctrine. Early Sunnis traditionally held that the political leader must come from the tribe of Muhammad—namely, the Quraysh tribe . The Zaydis narrowed the political claims of Ali's supporters, claiming that not just any descendant of Ali would be eligible to lead the Muslim community ( ummah ) but only those males directly descended from Muhammad through the union of Ali and Fatimah . But during the Abbasid revolts, other Shia, who came to be known as Imamiyyah (followers of the Imams), followed the theological school of Imam Ja'far al-Sadiq , himself the great great grandson of Muhammad's son-in-law Imam Ali. They asserted a more exalted religious role for Imams and insisted that, at any given time, whether in power or not, a single male descendant of Ali and Fatimah was the divinely appointed Imam and the sole authority, in his time, on all matters of faith and law. To those Shia, love of the imams and of their persecuted cause became as important as belief in God's oneness and the mission of Muhammad.
Later most of the Shia, including Twelver and Ismaili , became Imamis. Imami Shia believe that Imams are the spiritual and political successors to Muhammad. Imams are human individuals who not only rule over the community with justice, but also are able to keep and interpret the divine law and its esoteric meaning . The words and deeds of Muhammad and the imams are a guide and model for the community to follow; as a result, they must be free from error and sin, and must be chosen by divine decree, or nass , through Muhammad.
According to this view, there is always an Imam of the Age, who is the divinely appointed authority on all matters of faith and law in the Muslim community. Ali was the first imam of this line, the rightful successor to Muhammad, followed by male descendants of Muhammad through his daughter Fatimah.
This difference between following either the Ahl al-Bayt (Muhammad's family and descendants) or Caliph Abu Bakr has shaped Shia and non-Shia views on some of the Quranic verses, the hadith (narrations from Muhammad) and other areas of Islam. For instance, the collection of hadith venerated by Shia Muslims is centered on narrations by members of the Ahl al-Bayt and their supporters, while some hadith by narrators not belonging to or supporting the Ahl al-Bayt are not included. Those of Abu Hurairah , for example, Ibn Asakir in his Ta'rikh Kabir and Muttaqi in his Kanzu'l-Umma report that Caliph Umar lashed him, rebuked him and forbade him to narrate hadith from Muhammad. Umar said: "Because you narrate hadith in large numbers from the Holy Prophet, you are fit only for attributing lies to him. (That is, one expects a wicked man like you to utter only lies about the Holy Prophet.) So you must stop narrating hadith from the Prophet; otherwise, I will send you to the land of Dus." (A clan in Yemen, to which Abu Huraira belonged.) According to Sunnis, Ali was the fourth successor to Abu Bakr, while the Shia maintain that Ali was the first divinely sanctioned "Imam", or successor of Muhammad. The seminal event in Shia history is the martyrdom in 680 CE at the Battle of Karbala of Ali's son Hussein ibn Ali, who led a non-allegiance movement against the defiant caliph (71 of Hussein's followers were killed as well). Hussein came to symbolize resistance to tyranny.
It is believed in Twelver and Ismaili Shia Islam that 'aql , divine wisdom, was the source of the souls of the prophets and imams and gave them esoteric knowledge called ḥikmah and that their sufferings were a means of divine grace to their devotees. Although the imam was not the recipient of a divine revelation , he had a close relationship with God, through which God guides him, and the imam, in turn, guides the people. Imamate , or belief in the divine guide, is a fundamental belief in the Twelver and Ismaili Shia branches and is based on the concept that God would not leave humanity without access to divine guidance.
Imam of the time, last Imam of the Shia
The Mahdi is the prophesied redeemer of Islam who will rule for seven, nine or nineteen years (according to differing interpretations) before the Day of Judgment and will rid the world of evil. According to Islamic tradition, the Mahdi's tenure will coincide with the Second Coming of Jesus Christ (Isa), who is to assist the Mahdi against the Masih ad-Dajjal (literally, the "false Messiah" or Antichrist). Jesus, who is considered the Masih (Messiah) in Islam, will descend at the point of a white arcade, east of Damascus, dressed in yellow robes with his head anointed. He will then join the Mahdi in his war against the Dajjal, where Jesus will slay Dajjal and unite mankind.
The Shia Islamic faith is vast and inclusive of many different groups. Shia theological beliefs and religious practises, such as prayers, slightly differ from the Sunnis'. While all Muslims pray five times daily, Shias have the option of combining Dhuhr with Asr and Maghrib with Isha' , as there are three distinct times mentioned in the Quran. The Sunnis tend to combine only under certain circumstances.   Shia Islam embodies a completely independent system of religious interpretation and political authority in the Muslim world.   The original Shia identity referred to the followers of Imam Ali, and Shia theology was formulated in the 2nd century AH, or after Hijra (8th century CE). The first Shia governments and societies were established by the end of the 3rd century AH/9th century CE. The 4th century AH /10th century CE has been referred to by Louis Massignon as "the Shiite Ismaili century in the history of Islam".
The Shia believe that the status of Ali is supported by numerous hadith, including the Hadith of the pond of Khumm , Hadith of the two weighty things , Hadith of the pen and paper , Hadith of the invitation of the close families , and Hadith of the Twelve Successors . In particular, the Hadith of the Cloak is often quoted to illustrate Muhammad's feeling towards Ali and his family by both Sunni and Shia scholars. Shias prefer hadith attributed to the Ahl al-Bayt and close associates, and have their own separate collection of hadiths.
Profession of faith
The Shia version of the Shahada , the Islamic profession of faith, differs from that of the Sunni. The Sunni Shahada states There is no god except Allah, Muhammad is the messenger of Allah , but to this the Shia append Ali is the Wali (custodian) of God , علي ولي الله . This phrase embodies the Shia emphasis on the inheritance of authority through Muhammad's lineage. The three clauses of the Shia Shahada thus address tawhid (the unity of God ), nubuwwah (the prophethood of Muhammad), and imamah (imamate, the leadership of the faith).
Ismah is the concept of infallibility or "divinely bestowed freedom from error and sin" in Islam. Muslims believe that Muhammad and other prophets in Islam possessed ismah . Twelver and Ismaili Shia Muslims also attribute the quality to Imams as well as to Fatimah, daughter of Muhammad, in contrast to the Zaidi, who do not attribute 'ismah to the Imams. Though initially beginning as a political movement, infallibility and sinlessness of the imams later evolved as a distinct belief of (non-Zaidi) Shiism.
According to Shia theologians, infallibility is considered a rational necessary precondition for spiritual and religious guidance. They argue that since God has commanded absolute obedience from these figures they must only order that which is right. The state of infallibility is based on the Shia interpretation of the verse of purification .  Thus, they are the most pure ones, the only immaculate ones preserved from, and immune to, all uncleanness. It does not mean that supernatural powers prevent them from committing a sin, but due to the fact that they have absolute belief in God, they refrain from doing anything that is a sin. 
They also have a complete knowledge of God's will. They are in possession of all knowledge brought by the angels to the prophets ( nabi ) and the messengers ( rasul ). Their knowledge encompasses the totality of all times. They thus act without fault in religious matters. Shias regard Ali as the successor of Muhammad not only ruling over the community in justice, but also interpreting Islamic practices and its esoteric meaning. Hence he was regarded as being free from error and sin (infallible), and appointed by God by divine decree ( nass ) to be the first Imam.  Ali is known as "perfect man" ( al-insan al-kamil ) similar to Muhammad, according to Shia viewpoint. 
The Occultation is a belief in some forms of Shia Islam that a messianic figure , a hidden imam known as the Mahdi , will one day return and fill the world with justice. According to the Twelver Shia, the main goal of the Mahdi will be to establish an Islamic state and to apply Islamic laws that were revealed to Muhammad. 
Some Shia, such as the Zaidi and Nizari Ismaili, do not believe in the idea of the Occultation. The groups which do believe in it differ as to which lineage of the Imamate is valid, and therefore which individual has gone into occultation. They believe there are many signs that will indicate the time of his return.
Twelver Shia Muslims believe that the Mahdi (the twelfth imam , Muhammad al-Mahdi ) is already on Earth, is in occultation and will return at the end of time. Fatimid/ Bohra/ Dawoodi Bohra believe the same but for their 21st Tayyib , whereas Sunnis believe the future Mahdi has not yet arrived on Earth. 
Historians dispute the origin of Shia Islam , with many Western scholars positing that Shiism began as a political faction rather than a truly religious movement.  Other scholars disagree, considering this concept of religious-political separation to be an anachronistic application of a Western concept. 
Following the Battle of Karbala (680 AD), as various Shia-affiliated groups diffused in the emerging Islamic world, several nations arose based on a Shia leadership or population.
- Idrisids (788 to 985 CE): a Zaydi dynasty in what is now Morocco
- Uqaylids (990 to 1096 CE): a Shia Arab dynasty with several lines that ruled in various parts of Al-Jazira , northern Syria and Iraq.
- Buyids (934–1055 CE): at its peak consisted of large portions of modern Iraq and Iran.
- Ilkhanate (1256–1335): a Mongol khanate established in Persia in the 13th century, considered a part of the Mongol Empire . The Ilkhanate was based, originally, on Genghis Khan 's campaigns in the Khwarezmid Empire in 1219–1224, and founded by Genghis's grandson, Hulagu , in territories which today comprise most of Iran, Iraq, Afghanistan, Turkmenistan, Armenia, Azerbaijan, Georgia, Turkey, and Pakistan. The Ilkhanate initially embraced many religions, but was particularly sympathetic to Buddhism and Christianity. Later Ilkhanate rulers, beginning with Ghazan in 1295, embraced Islam his brother Öljaitü promoted Shia Islam.
- Naubat Khan accepted Islam under the Guidance of Mughal General Bairam Khan 's son Abdul Rahim Khan-I-Khana .
- Bahmanis (1347–1527 CE): a Shia Muslim state of the Deccan in southern India and one of the great medieval Indian kingdoms.  Bahmanid Sultanate was the first independent Islamic Kingdom in South India. 
- Fatimids (909–1171 CE): Controlled much of North Africa, the Levant , parts of Arabia and Mecca and Medina . The group takes its name from Fatima, Muhammad's daughter, from whom they claim descent.
- In 909 CE the Shiite military leader Abu Abdallah, overthrew the Sunni ruler in Northern Africa; which began the Fatimid regime.
A major turning point in Shia history was the Safavid dynasty (1501–1736) in Persia. This caused a number of changes in the Muslim world:
- The ending of the relative mutual tolerance between Sunnis and Shias that existed from the time of the Mongol conquests onwards and the resurgence of antagonism between the two groups.
- Initial dependence of Shiite clerics on the state followed by the emergence of an independent body of ulama capable of taking a political stand different from official policies.
- The growth in importance of Iranian centers of religious learning and change from Twelver Shiism being a predominantly Arab phenomenon.
- The growth of the Akhbari School which preached that only the Quran, hadith are to be bases for verdicts, rejecting the use of reasoning.
With the fall of the Safavids, the state in Persia – including the state system of courts with government-appointed judges ( qadis ) – became much weaker. This gave the Sharia courts of mujtahids an opportunity to fill the legal vacuum and enabled the ulama to assert their judicial authority. The Usuli School also increased in strength at this time.
According to Shia Muslims, one of the lingering problems in estimating Shia population is that unless Shia form a significant minority in a Muslim country, the entire population is often listed as Sunni. The reverse, however, has not held true, which may contribute to imprecise estimates of the size of each sect. For example, the 1926 rise of the House of Saud in Arabia brought official discrimination against Shia.  Shiites are estimated to be 21–35% of the Muslim population in South Asia , although the total number is difficult to estimate due to that reason. It is variously estimated that 10–20%     of the world's Muslims are Shia. They may number up to 200 million as of 2009. 
The Shia majority countries are Iran , Iraq , Azerbaijan , and Bahrain .   They also form the plurality (the largest group, but not the majority) in Lebanon. Shias constitute 36.3% of entire local population and 38.6% of the local Muslim population of the Middle East.
Shia Muslims constitute 27-35% of the population in Lebanon, and as per some estimates from 35%   to over 35-40% of the population in Yemen,  30%-35% of the citizen population in Kuwait (no figures exist for the non-citizen population),  over 20% in Turkey,   5–20% of the population in Pakistan,   and 10-19% of Afghanistan 's population.  
Saudi Arabia hosts a number of distinct Shia communities, including the Twelver Baharna in the Eastern Province and Nakhawila of Medina, and the Ismaili Sulaymani and Zaidiyyah of Najran . Estimations put the number of Shiite citizens at 2-4 million, accounting for roughly 15% of the local population. 
Significant Shia communities exist in the coastal regions of West Sumatra and Aceh in Indonesia (see Tabuik ).  The Shia presence is negligible elsewhere in Southeast Asia, where Muslims are predominantly Shafi'i Sunnis.
A significant Shia minority is present in Nigeria , made up of modern-era converts to a Shia movement centered around Kano and Sokoto states.    Several African countries like Kenya,  South Africa,  Somalia,  etc. hold small minority populations of various Shia denominations, primarily descendants of immigrants from South Asia during the colonial period, such as the Khoja . 
List of Nations for which the Shia population may be estimated
|Country||Shia population  ||Percent of Muslim population that is Shia  ||Percent of global Shia population  ||Minimum estimate/claim||Maximum estimate/claim|
|Iran||74,000,000 – 78,000,000||90–95||37–40||78,661,551  |
|Pakistan||17,000,000 – 26,000,000||10–15||10-15||43,250,000  – 57,666,666  |
|India||17,000,000 – 26,000,000||10–15||9–14||40,000,000  – 50,000,000. |
|Iraq||19,000,000 – 22,000,000||65–70||11–12|
|Yemen||8,000,000 – 10,000,000||35–40||~5|
|Turkey||7,000,000 – 11,000,000||10–15||4–6||22 million |
|Azerbaijan||5,000,000 – 7,000,000||65–75||3-4||8.16 million,  85% of total population |
|Afghanistan||3,000,000 – 4,000,000||10–15||~2||6.1 million,  15–19% of total population |
|Syria||3,000,000 – 4,000,000||15–20||~2|
|Saudi Arabia||2,000,000 – 4,000,000||10–15||1-2|
|Nigeria||<4,000,000||<5||<2||22-25 million |
|Lebanon||1,000,000 – 2,000,000||45-55||<1||Estimated, no official census.  50-55%   |
|Kuwait||500,000 - 700,000||20-25||<1||30%-35% of 1.2m Muslims (citizen only) |
|Germany||400,000 – 600,000||10–15||<1|
|Bahrain||400,000 – 500,000||65–70||<1||100,000 (66%  of citizen population )||200,000 (70%  of citizen population )|
|United Arab Emirates||300,000 – 400,000||10||<1|
|United States||200,000 – 400,000||10–15||<1|
|Oman||100,000 – 300,000||5–10||<1||948,750 |
|United Kingdom||100,000 – 300,000||10–15||<1|
The history of Sunni-Shia relations has often involved violence, dating back to the earliest development of the two competing sects. At various times Shia groups have faced persecution.      
Militarily established and holding control over the Umayyad government, many Sunni rulers perceived the Shia as a threat – to both their political and their religious authority.  The Sunni rulers under the Umayyads sought to marginalize the Shia minority, and later the Abbasids turned on their Shia allies and imprisoned, persecuted, and killed them. The persecution of the Shia throughout history by Sunni co-religionists has often been characterized by brutal and genocidal acts. Comprising only about 10–15% of the entire Muslim population, the Shia remain a marginalized community to this day in many Sunni Arab dominant countries without the rights to practice their religion and organize. 
In 1514 the Ottoman sultan , Selim I , ordered the massacre of 40,000 Anatolian Shia.  According to Jalal Al-e-Ahmad , "Sultan Selim I carried things so far that he announced that the killing of one Shiite had as much otherworldly reward as killing 70 Christians." 
Shia, celebrate the following annual holidays:
- Eid ul-Fitr , which marks the end of fasting during the month of Ramadan
- Eid al-Adha , which marks the end of the Hajj or pilgrimage to Mecca
The following days are some of the most important holidays observed by Shia Muslims:
- Eid al-Ghadeer , which is the anniversary of the Ghadir Khum, the occasion when Muhammad announced Ali's Imamate before a multitude of Muslims.  Eid al-Ghadeer is held on the 18th of Dhu al-Hijjah.
- The Mourning of Muharram and the Day of Ashura for Shia commemorates Husayn ibn Ali's martyrdom. Husayn was a grandson of Muhammad who was killed by Yazid ibn Muawiyah. Ashurah is a day of deep mourning which occurs on the 10th of Muharram .
- Arba'een commemorates the suffering of the women and children of Husayn ibn Ali's household. After Husayn was killed, they were marched over the desert, from Karbala (central Iraq) to Shaam ( Damascus , Syria). Many children (some of whom were direct descendants of Muhammad) died of thirst and exposure along the route. Arbaein occurs on the 20th of Safar , 40 days after Ashurah.
- Mawlid , Muhammad's birth date. Unlike Sunni Muslims, who celebrate the 12th of Rabi' al-awwal as Muhammad's birthday or deathday (because they assert that his birth and death both occur in this week), Shia Muslims celebrate Muhammad's birthday on the 17th of the month, which coincides with the birth date of the sixth imam, Ja'far al-Saadiq.  Wahhabis do not celebrate Muhammad's birthday, believing that such celebrations constitute a bid‘ah . 
- Fatimah 's birthday on 20th of Jumada al-Thani . This day is also considered as the "'women and mothers' day".
- Ali 's birthday on 13th of Rajab .
- Mid-Sha'ban is the birth date of the 12th and final Twelver imam, Muhammad al-Mahdi . It is celebrated by Shia Muslims on the 15th of Sha'aban .
- Laylat al-Qadr , anniversary of the night of the revelation of the Quran.
- Eid al-Mubahila celebrates a meeting between the Ahl al-Bayt (household of Muhammad) and a Christian deputation from Najran. Al-Mubahila is held on the 24th of Dhu al-Hijjah.
The four holiest sites to Muslims are Mecca (Al-Haram Mosque), Medina (Al-Nabbawi Mosque), Jerusalem (Al-Aqsa Mosque), and Kufa (Kufa Mosque). In addition for Shias, the Imam Husayn Shrine , Al Abbas Mosque in Karbala, and Imam Ali Mosque in Najaf are also highly revered.
Other venerated sites include Wadi-us-Salaam cemetery in Najaf , Al-Baqi' cemetery in Medina, Imam Reza shrine in Mashhad , Kadhimiya Mosque in Kadhimiya , Al-Askari Mosque in Samarra , Sahla Mosque and Great Mosque of Kufa in Kufa and several other sites in the cities of Qom , Susa and Damascus.
Most of the Shia holy places in Saudi Arabia have been destroyed by the warriors of the Ikhwan , the most notable being the tombs of the Imams in the Al-Baqi' cemetery in 1925.  In 2006, a bomb destroyed the shrine of Al-Askari Mosque. 
The Shia belief throughout its history split over the issue of the Imamate. The largest branch are the Twelvers, followed by the Zaidi and Ismaili. All three groups follow a different line of Imamate.
Twelver Shia or the Ithnā'ashariyyah' is the largest branch of Shia Islam, and the term Shia Muslim often refers to the Twelvers by default. The term Twelver is derived from the doctrine of believing in twelve divinely ordained leaders, known as The Twelve Imams . Twelver Shia are also known as Imami or Ja'fari , originated from the name of the 6th Imam, Ja'far al-Sadiq, who elaborated the twelver jurisprudence. 
- Monotheism , God is one and unique.
- Justice , the concept of moral rightness based on ethics, fairness, and equity, along with the punishment of the breach of said ethics.
- Prophethood , the institution by which God sends emissaries, or prophets, to guide mankind.
- Leadership , a divine institution which succeeded the institution of Prophethood. Its appointees (imams) are divinely appointed.
- Last Judgment , God's final assessment of humanity.
More specifically, these principles are known as Usul al-Madhhab (principles of the Shia sect) according to Twelver Shias which differ from Daruriyat al-Din (Necessities of Religion) which are principles in order for one to be a Muslim. The Necessities of Religion do not include Leadership (Imamah) as it is not a requirement in order for one to be recognized as a Muslim. However, this category, according to Twelver scholars like Ayatollah al-Khoei, does include belief in God, Prophethood, the Day of Resurrection and other "necessities" (like belief in angels). In this regard, Twelver Shias draw a distinction in terms of believing in the main principles of Islam on the one hand, and specifically Shia doctrines like Imamah on the other.
Besides the Quran which is common to all Muslims, the Shiah derive guidance from books of traditions ("ḥadīth") attributed to Muhammad and the Twelve Imams. Below is a list of some of the most prominent of these books:
- Nahj al-Balagha by Ali ibn Abi Talib - the most famous collection of sermons, letters & narration by the first Imam regarded by Shias
- al-Kafi by Muhammad ibn Ya'qub al-Kulayni 
- Wasa'il al-Shi'ah by al-Hurr al-Amili
The Twelve Imams
The Twelve Imams are the spiritual and political successors to Muhammad for the Twelvers. According to the theology of Twelvers, the successor of Muhammad is an infallible human individual who not only rules over the community with justice but also is able to keep and interpret the divine law and its esoteric meaning. The words and deeds of Muhammad and the imams are a guide and model for the community to follow; as a result, they must be free from error and sin, and Imams must be chosen by divine decree, or nass , through Muhammad. Each imam was the son of the previous imam, with the exception of Hussein ibn Ali, who was the brother of Hasan ibn Ali. The twelfth and final imam is Muhammad al-Mahdi, who is believed by the Twelvers to be currently alive and in occultation.
The Twelver jurisprudence is called Ja'fari jurisprudence . In this jurisprudence Sunnah is considered to be the oral traditions of Muhammad and their implementation and interpretation by the twelve Imams. There are three schools of Ja'fari jurisprudence: Usuli, Akhbari, and Shaykhi . The Usuli school is by far the largest of the three. Twelver groups that do not follow Ja'fari jurisprudence include Alevi , Bektashi , and Qizilbash .
In Ja'fari jurisprudence, there are ten ancillary pillars, known as Furu' ad-Din , which are as follows: 
- Alms giving
- Directing others towards good
- Directing others away from evil
- Alms giving ( One Fifth ) (20% tax on yearly earnings after deduction of household and commercial expenses.)
- Love those who are in God's path
- Disassociation with those who oppose God
According to Twelvers, defining and interpretation of Islamic jurisprudence is the responsibility of Muhammad and the twelve Imams. As the 12th Imam is in occultation, it is the duty of clerics to refer to the Islamic literature such as the Quran and hadith and identify legal decisions within the confines of Islamic law to provide means to deal with current issues from an Islamic perspective. In other words, Twelver clerics provide Guardianship of the Islamic Jurisprudence, which was defined by Muhammad and his twelve successors. This process is known as Ijtihad and the clerics are known as Marja ' , meaning reference. The labels Allamah and Ayatollah are in use for Twelver clerics.
Zaidiyya , Zaidism or Zaydi is a Shia school named after Zayd ibn Ali . Followers of the Zaidi fiqh are called Zaidis (or occasionally Fivers ). However, there is also a group called Zaidi Wasītīs who are Twelvers (see below). Zaidis constitute roughly 42–47% of the population of Yemen.  
The Zaydis, Twelvers, and Ismailis all recognize the same first four Imams; however, the Zaidis recognize Zayd ibn Ali as the fifth. After the time of Zayd ibn Ali, the Zaidis recognized that any descendant of Hasan ibn Ali or Hussein ibn Ali could be imam after fulfilling certain conditions.  Other well-known Zaidi Imams in history were Yahya ibn Zayd, Muhammad al-Nafs al-Zakiyya and Ibrahim ibn Abdullah. In matters of Islamic jurisprudence , the Zaydis follow Zayd ibn Ali's teachings which are documented in his book Majmu'l Fiqh (in Arabic : مجموع الفِقه ). Al-Hadi ila'l-Haqq Yahya , founder of the Zaydi state in Yemen, instituted elements of the jurisprudential tradition of the Sunni Muslim jurist Abū Ḥanīfa , and as a result, Zaydi jurisprudence today continues somewhat parallel to that of the Hanafis .
The Zaidi doctrine of Imamah does not presuppose the infallibility of the imam nor that the Imams receive divine guidance. Zaidis also do not believe that the Imamate must pass from father to son but believe it can be held by any Sayyid descended from either Hasan ibn Ali or Hussein ibn Ali (as was the case after the death of Hasan ibn Ali). Historically, Zaidis held that Zayd was the rightful successor of the 4th imam since he led a rebellion against the Umayyads in protest of their tyranny and corruption. Muhammad al-Baqir did not engage in political action, and the followers of Zayd believed that a true imam must fight against corrupt rulers.
A Zaydi state was established in Gilan , Deylaman and Tabaristan (northern Iran) in 864 C.E. by the Alavids ;  it lasted until the death of its leader at the hand of the Samanids in 928 C.E. Roughly forty years later the state was revived in Gilan and survived under Hasanid leaders until 1126 C.E. Afterwards, from the 12th to 13th centuries, the Zaydis of Deylaman, Gilan and Tabaristan then acknowledged the Zaydi Imams of Yemen or rival Zaydi Imams within Iran. 
The Buyids were initially Zaidi  as were the Banu Ukhaidhir rulers of al-Yamama in the 9th and 10th centuries.  The leader of the Zaydi community took the title of Caliph. As such, the ruler of Yemen was known as the Caliph, al-Hadi Yahya bin al-Hussain bin al-Qasim ar-Rassi Rassids (a descendant of Hasan ibn Ali the son of Ali) who, at Sa'dah , in 893-7 CE, founded the Zaydi Imamate, and this system continued until the middle of the 20th century, when the revolution of 1962 CE deposed the Zaydi Imam. The founding Zaidism of Yemen was of the Jarudiyya group; however, with increasing interaction with Hanafi and Shafi'i rites of Sunni Islam , there was a shift from the Jarudiyya group to the Sulaimaniyya, Tabiriyya, Butriyya or Salihiyya groups.  Zaidis form the second dominant religious group in Yemen. Currently, they constitute about 40–45% of the population in Yemen. Ja'faris and Isma'ilis are 2–5%.  In Saudi Arabia, it is estimated that there are over 1 million Zaydis (primarily in the western provinces).
Currently the most prominent Zaydi movement is Houthis movement, known by the name of Shabab Al Mu'mineen (Believing Youth) or AnsarAllah (Partisans of God). In 2014–2015 Houthis took over the government in Sana'a , which led to the fall of the Saudi Arabian-backed government of Abd Rabbuh Mansur Hadi .  Houthis and their allies gained control of a significant part of Yemen's territory and were resisting the Saudi Arabian-led intervention in Yemen seeking to restore Hadi in power. Both the Houthis and the Saudi Arabian-led coalition were being attacked by the Islamic State of Iraq and the Levant .  
Ismailis gain their name from their acceptance of Isma'il ibn Jafar as the divinely appointed spiritual successor ( Imam ) to Ja'far al-Sadiq, wherein they differ from the Twelvers, who accept Musa al-Kadhim, younger brother of Isma'il, as the true Imam.
After the death or Occultation of Muhammad ibn Ismaill in the 8th century, the teachings of Ismailism further transformed into the belief system as it is known today, with an explicit concentration on the deeper, esoteric meaning ( bāṭin ) of the faith. With the eventual development of Twelverism into the more literalistic ( zahir ) oriented Akhbari and later Usuli schools of thought, Shiaism developed in two separate directions: the metaphorical Ismailli group focusing on the mystical path and nature of God and the divine manifestation in the personage of the "Imam of the Time" as the "Face of God", with the more literalistic Twelver group focusing on divine law ( sharī'ah ) and the deeds and sayings ( sunnah ) of Muhammad and his successors (the Ahlu l-Bayt ), who as A'immah were guides and a light to God. 
Though there are several sub-groupings within the Ismailis, the term in today's vernacular generally refers to The Shia Imami Ismaili Muslim ( Nizari community), generally known as the Ismailis, who are followers of the Aga Khan and the largest group among the Ismailiyyah. Another community which falls under the Isma'il's are the Dawoodi Bohras , led by a Da'i al-Mutlaq as representative of a hidden imam. While there are many other branches with extremely differing exterior practices, much of the spiritual theology has remained the same since the days of the faith's early Imams. In recent centuries Ismailis have largely been an Indo-Iranian community,  but they are found in India, Pakistan, Syria, Palestine, Saudi Arabia,  Yemen, China,  Jordan, Uzbekistan, Tajikistan, Afghanistan, East Africa and South Africa, and have in recent years emigrated to Europe, Australia, New Zealand, and North America.
After the death of Isma'il ibn Jafar, many Ismailis believed that one day the messianic Mahdi, whom they believed to be Muhammad ibn Ismail, would return and establish an age of justice. One group included the violent Qarmatians , who had a stronghold in Bahrain. In contrast, some Ismailis believed the Imamate did continue, and that the Imams were in occultation and still communicated and taught their followers through a network of dawah "Missionaries".
In 909, Ubayd Allah al-Mahdi Billah , a claimant to the Ismaili Imamate, established the Fatimid Caliphate. During this period, three lineages of imams formed. The first branch, known today as the Druze , began with Al-Hakim bi-Amr Allah . Born in 386 AH (985), he ascended as ruler at the age of eleven. The typical religiously tolerant Fatimid Empire saw much persecution under his reign. When in 411 AH (1021) his mule returned without him, soaked in blood, a religious group that was forming in his lifetime broke off from mainstream Ismailism and did not acknowledge his successor. Later to be known as the Druze, they believe al-Hakim to be the incarnation of God and the prophesied Mahdi who would one day return and bring justice to the world.  The faith further split from Ismailism as it developed very unusual doctrines which often class it separately from both Ismailiyyah and Islam.
The second split occurred following the death of Ma'ad al-Mustansir Billah in 487 AH (1094). His rule was the longest of any caliph in any Islamic empire. Upon his passing away, his sons, Nizar the older, and Al-Musta'li , the younger, fought for political and spiritual control of the dynasty. Nizar was defeated and jailed, but according to Nizari tradition, his son escaped to Alamut , where the Iranian Ismaili had accepted his claim. From here on, the Nizari Ismaili community has continued with a present, living Imam.
The Mustaali line split again between the Taiyabi (Dawoodi Bohra is its main branch) and the Hafizi . The former claim that At-Tayyib Abi l-Qasim (son of Al-Amir bi-Ahkami l-Lah ) and the imams following him went into a period of anonymity ( Dawr-e-Satr ) and appointed a Da'i al-Mutlaq to guide the community, in a similar manner as the Ismaili had lived after the death of Muhammad ibn Ismail. The latter (Hafizi) claimed that the ruling Fatimid Caliph was the Imam, and they died out with the fall of the Fatimid Empire.
Ismailis have categorized their practices which are known as seven pillars :
The Shahada (profession of faith) of the Shia differs from that of Sunnis due to mention of Ali. 
The Nizaris place importance on a scholarly institution because of the existence of a present Imam. The Imam of the Age defines the jurisprudence, and his guidance may differ with Imams previous to him because of different times and circumstances. For Nizari Ismailis, the Imam is Karim al-Husayni Aga Khan IV . The Nizari line of Imams has continued to this day as an unending line.
Divine leadership has continued in the Bohra branch through the institution of the "Unrestricted Missionary" Dai . According to Bohra tradition, before the last Imam, At-Tayyib Abi l-Qasim, went into seclusion, his father, the 20th Al-Amir bi-Ahkami l-Lah, had instructed Al-Hurra Al-Malika the Malika ( Queen consort ) in Yemen to appoint a vicegerent after the seclusion – the Unrestricted Missionary , who as the Imam's vicegerent has full authority to govern the community in all matters both spiritual and temporal while the lineage of Mustaali-Tayyibi Imams remains in seclusion (Dawr-e-Satr). The three branches of the Mustaali, the Alavi Bohra , Sulaimani Bohra and Dawoodi Bohra , differ on who the current Unrestricted Missionary is.
Doctrine about necessity of acquiring knowledge
According to Allameh Muzaffar, Allah gives humans the faculty of reason and argument. Also, Allah orders humans to spend time thinking carefully on creation while he refers to all creations as his signs of power and glory. These signs encompass all of the universe. Furthermore, there is a similarity between humans as the little world and the universe as the large world. Allah does not accept the faith of those who follow him without thinking and only with imitation, but also Allah blames them for such actions. In other words, humans have to think about the universe with reason and intellect, a faculty bestowed on us by Allah. Since there is more insistence on the faculty of intellect among Shia, even evaluating the claims of someone who claims prophecy is on the basis of intellect.  
Doctrine concerning Du'a
Praying or Du’a in Shia has an important place as Muhammad described it as a weapon of the believer. In fact, Du’a considered as something that is a feature of Shia community in a sense. Of course, performing Du’a in Shia has a special ritual. Because of this, there are many books written on the conditions of praying among Shia. Most of ad’ayieh transferred from Muhammad's household and then by many books in which we can observe the authentic teachings of Muhammad and his household according to Shia. The leaderships of Shia always invited their followers to recite Du’a. For instance, Ali has considered with the subject of Du’a because of his leadership in monotheism.  
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