Guerrilla warfare is a form of irregular warfare in which small groups of combatants, such as paramilitary personnel, armed civilians, or irregulars, use military tactics including ambushes, sabotage, raids, petty warfare, hit-and-run tactics, and mobility, to fight a larger and less-mobile traditional military. Guerrilla groups are a type of violent non-state actor.

The Spanish word "guerrilla" is the diminutive form of "guerra" ("war"). The term became popular during the early-19th century Peninsular War, when the Spanish and Portuguese people rose against the Napoleonic troops and fought against a highly superior army using the guerrilla strategy. In correct Spanish usage, a person who is a member of a "guerrilla" unit is a "guerrillero" ([ɣeriˈʎeɾo]) if male, or a "guerrillera" ([ɣeriˈʎeɾa]) if female.

The term "guerrilla" was used in English as early as 1809 to refer to the fighters (e.g., "The town was taken by the guerrillas"), and also (as in Spanish) to denote a group or band of such fighters. However, in most languages guerrilla still denotes the specific style of warfare. The use of the diminutive evokes the differences in number, scale, and scope between the guerrilla army and the formal, professional army of the state.

Guerrilla warfare is a type of asymmetric warfare: competition between opponents of unequal strength.^[1] It is also a type of irregular warfare: that is, it aims not simply to defeat an enemy, but to win popular support and political influence, to the enemy's cost.^[2] Accordingly, guerrilla strategy aims to magnify the impact of a small, mobile force on a larger, more-cumbersome one.^[3] If successful, guerrillas weaken their enemy by attrition, eventually forcing them to withdraw.

Tactically, guerrillas usually avoid confrontation with large units and formations of enemy troops, but seek and attack small groups of enemy personnel and resources to gradually deplete the opposing force while minimizing their own losses. The guerrilla prizes mobility, secrecy, and surprise, organizing in small units and taking advantage of terrain that is difficult for larger units to use. For example, Mao Zedong summarized basic guerrilla tactics at the beginning of the Chinese "Second Revolutionary Civil War" as:

At least one author credits the ancient Chinese work The Art of War with inspiring Mao's tactics.^{[5] []} In the 20th century, other communist leaders, including North Vietnamese Ho Chi Minh, often used and developed guerrilla warfare tactics, which provided a model for their use elsewhere, leading to the Cuban "foco" theory and the anti-Soviet Mujahadeen in Afghanistan.^[5]

In addition to traditional military methods, guerrilla groups may rely also on destroying infrastructure, using improvised explosive devices, for example. They typically also rely on logistical and political support from the local population and foreign backers, are often embedded within it (thereby using the population as a human shield), and many guerrilla groups are adept at public persuasion through propaganda.^[6] Many guerrilla movements today also rely heavily on children as combatants, scouts, porters, spies, informants, and in other roles,^[7] which has drawn international condemnation^[8] (although many states also recruit children into their armed forces).^[9] Guerrilla groups also use displace people to solidify power or politically destabilize an adversary.

There is no commonly accepted definition of "terrorism",^[10][11][12] and the term is frequently used as a political tactic by belligerents (most often by governments in power) to denounce opponents whose status as terrorists is disputed.^[13][14]

Contrary to some terrorist groups, guerrillas usually work in open positions as armed units, try to hold and seize land, do not refrain from fighting enemy military force in battle and usually apply pressure to control or dominate territory and population. While the primary concern of guerrillas is the enemy's active military units, terrorists largely are concerned with non-military agents and target mostly civilians. Guerrilla forces principally fight in accordance with the law of war (jus in bello). In this sense, they respect the rights of innocent civilians by refraining from targeting them. According to the Ankara Center for Crisis and Policy Studies, terrorists do not limit their actions and terrorise civilians by putting fear in people's hearts and even kill innocent foreigners in the country.^[15]

Irregular warfare, based on elements later characteristic of modern guerrilla warfare, has existed throughout the battles of many ancient civilizations. The growth of guerrilla warfare in the 20th century was inspired in part by theoretical works on guerrilla warfare, starting with the Manual de Guerra de Guerrillas by Matías Ramón Mella written in the 19th century and, more recently, Mao Zedong's On Guerrilla Warfare, Che Guevara's Guerrilla Warfare ^[46] , and Lenin's text of the same name ^[47] , all written after the successful revolutions carried by them in China, Cuba and Russia, respectively. Those texts characterized the tactic of guerrilla warfare as, according to Che Guevara's text, being

The Chinese general and strategist Sun Tzu, in his The Art of War (6th century BC), was the earliest to propose the use of guerrilla warfare.^[17] This directly inspired the development of modern guerrilla warfare.^[18] Guerrilla tactics were presumably employed by prehistoric tribal warriors against enemy tribes.^[19] Evidence of conventional warfare, on the other hand, did not emerge until 3100 BC in Egypt and Mesopotamia. Since the Enlightenment, ideologies such as nationalism, liberalism, socialism, and religious fundamentalism have played an important role in shaping insurgencies and guerrilla warfare. The Moroccan national hero Mohamed ben Abdelkrim el-Khattabi, along with his father, unified the Moroccan tribes under their control and took arms and resistance against the Spanish and French invaders of the early 20th century. For the first time in history, tunnel warfare was utilized alongside modern guerilla tactics which caused considerable damage and annoyance to both of the invading armies in Morocco. ^[20]

Quintus Fabius Maximus Verrucosus, widely regarded as the "father of guerrilla warfare",^[21] devised the Fabian strategy which was used to great effect against Hannibal Barca's army.^[22][23] The strategy would further influence guerrilla tactics into the modern era.^[21]

A counter-insurgency or counterinsurgency^[24] (COIN) operation involves actions taken by the recognised government of a nation to contain or quell an insurgency taken up against it.^[25] In the main, the insurgents seek to destroy or erase the political authority of the defending authorities in a population they seek to control, and the counter-insurgent forces seek to protect that authority and reduce or eliminate the supplanting authority of the insurgents. Counter-insurgency operations are common during war, occupation and armed rebellions. Counter-insurgency may be armed suppression of a rebellion, coupled with tactics such as "hearts and minds" designed to fracture the links between the insurgency and the population in which the insurgents move. Because it may be difficult or impossible to distinguish between an insurgent, a supporter of an insurgency who is a non-combatant, and entirely uninvolved members of the population, counter-insurgency operations have often rested on a confused, relativistic, or otherwise situational distinction between insurgents and non-combatants.

Theorists of counter-insurgency warfare have written extensively on the subject since the 1950s and 1960s but as early as the 1720s the third Marques of Santa Cruz de Marcenado (1684–1732) wrote that insurgencies were often the result of state failure and that the goal of those fighting the insurgents should be to seek the people's "heart and love".^[26] The two most influential of scholars of counter-insurgency have been Westerners whose job it had been to fight insurgents (often colonised people). Robert Thompson fought during the Malayan Emergency and David Galula fought during the Algerian War. Together these officers advocated multi-pronged strategies to win over the civilian population to the side of the counter-insurgent.

The widely distributed and influential work of Sir Robert Thompson, counter-insurgency expert of the Malayan Emergency, offers several such guidelines. Thompson's underlying assumption was that the counter-insurgent was committed to improving the rule of law and bettering local governance.^[27] Some governments, however, give such considerations short shrift. These governments are not interested in state-building and in extreme cases they have carried out counter-insurgency operations by using mass murder, genocide, terror, torture and execution. Historian Timothy Snyder has written, "In the guise of anti-partisan actions, the Germans killed perhaps three quarters of a million people, about 350,000 in Belarus alone, and lower but comparable numbers in Poland and Yugoslavia. The Germans killed more than a hundred thousand Poles when suppressing the Warsaw Uprising of 1944."^[28]

In the Vietnam War, the Americans "defoliated countless trees in areas where the communist North Vietnamese troops hid supply lines and conducted guerrilla warfare"^[29] (see Operation Ranch Hand). In the Soviet–Afghan War, the Soviets countered the U.S.–backed Mujahideen with a 'Scorched Earth' policy, driving over one third of the Afghan population into exile (over 5 million people), and carrying out widespread destruction of villages, granaries, crops, herds and irrigation systems, including the deadly and widespread mining of fields and pastures.^[30][31]

Some writers on counter-insurgency warfare emphasise the more turbulent nature of today's guerrilla warfare environment, where the clear political goals, parties and structures of such places as Vietnam, Malaya, and El Salvador are not as prevalent. These writers point to numerous guerrilla conflicts that centre around religious, ethnic or even criminal enterprise themes, and that do not lend themselves to the classic "national liberation" template.

The wide availability of the Internet has also caused changes in the tempo and mode of guerrilla operations in such areas as coordination of strikes, leveraging of financing, recruitment, and media manipulation. While the classic guidelines still apply, today's anti-guerrilla forces need to accept a more disruptive, disorderly and ambiguous mode of operation. According to David Kilcullen:

In the 1960s, the Marxist revolutionary Che Guevara developed the foco (Spanish: foquismo) theory of revolution in his book Guerrilla Warfare, based on his experiences during the 1959 Cuban Revolution. This theory was later formalised as "focal-ism" by Régis Debray. Its central principle is that vanguardism by cadres of small, fast-moving paramilitary groups can provide a focus for popular discontent against a sitting regime, and thereby lead a general insurrection. Although the original approach was to mobilise and launch attacks from rural areas, many foco ideas were adapted into urban guerrilla warfare movements.