A preemptive war is a war that is commenced in an attempt to repel or defeat a perceived imminent offensive or invasion, or to gain a strategic advantage in an impending (allegedly unavoidable) war shortly before that attack materializes. It is a war that preemptively 'breaks the peace'.
The term 'preemptive war' is sometimes confused with the term ' preventive war'. The difference is that a preventive war is launched to destroy the potential threat of the targeted party, when an attack by that party is not imminent or known to be planned. A preemptive war is launched in anticipation of immediate aggression by another party.  Most contemporary scholarship equates preventive war with aggression, and therefore argues that it is illegitimate. The waging of a preemptive war has less stigma attached than does the waging of a preventive war.
The initiation of armed conflict: that is being the first to 'break the peace' when no 'armed attack' has yet occurred, is not permitted by the UN Charter, unless authorized by the UN Security Council as an enforcement action. Some authors have claimed that when a presumed adversary first appears to be beginning confirmable preparations for a possible future attack, but has not yet actually attacked, that the attack has in fact 'already begun', however this opinion has not been upheld by the UN. 
Theory and practice of preemptive war
Prior to World War I
As early as 1625, Hugo Grotius characterized a state's right of self-defense to include the right to forestall an attack forcibly.  In 1685, the Scottish government conducted a preemptive strike against the Clan Campbell, called the Argyll Whigs.  In 1837, a certain legal precedent regarding preemptive wars was established in the Caroline affair when British forces in Canada crossed the United States border and killed several Canadian rebels and one American citizen who were preparing an offensive against the British in Canada. The United States rejected the legal ground of the Caroline case. In 1842, US Secretary of State Daniel Webster said that the necessity for forcible reaction must be "instant, overwhelming, leaving no choice of means, and no moment for deliberation." That formulation is part of the Caroline test, which "is broadly cited as enshrining the appropriate customary law standard."
World War I (1914–1918)
Austro-Hungarian Chief of the General Staff Franz Conrad von Hötzendorf argued for a preemptive war against Serbia in 1913. Serbia had the image of an aggressive and expansionist power and was seen as a threat to Austria-Hungary in Bosnia and Herzegovina. The assassination of Archduke Franz Ferdinand (June 1914) justified Austria-Hungary to attack, leading to World War I.
During the course of the destructive and costly World War I, for the first time in history, the concept of " the war to end war " began to be seriously considered.  As a further expression of that hope, upon the conclusion of the war, the League of Nations was formed. Its primary aim was to prevent war, as all signatories were required to agree to desist from the initiation of all wars, preemptive or otherwise. All of the victorious nations emerging out of World War I eventually signed the agreement, with the notable exception of the United States. 
League of Nations period (1919–1939)
During the 1920s, the League peaceably settled numerous international disputes, and was generally perceived as succeeding in its primary purpose. It was only in the 1930s that its effectiveness in preventing wars began to come into question. Such questions began to arise when it first became apparent in 1931 that it was incapable of halting aggression by Japan in Manchuria. In the Mukden Incident, Japan claimed to be fighting a 'defensive war' in Manchuria, attempting to 'preempt' supposedly aggressive Chinese intentions towards the Japanese. According to the Japanese, the Chinese had started the war by blowing up a railway line near Mukden, China. Therefore, the Chinese were the aggressors, and the Japanese were merely 'defending themselves'. A predominance of evidence has since indicated that the railway had in fact most probably been blown up by Japanese operatives. 
In 1933, the impotency of the League became more pronounced when notices were provided by Japan and Germany that they would be terminating their memberships in the League. Italy shortly followed suit and exited the League in 1937.  Soon, Italy and Germany also began engaging in militaristic campaigns designed to either enlarge their borders or to expand their sphere of military control, and the League was shown to be powerless to stop them.  The perceived impotency of the League was a contributing factor which eventually led to the full outbreak of World War II in 1939.  The start of World War II is generally dated from the event of Germany's invasion of Poland. It is noteworthy that Germany claimed at the time that its invasion of Poland was in fact a 'defensive war,' as it had allegedly been invaded by a group of Polish saboteurs, signaling a potentially larger invasion of Germany by Poland that was soon to be under way. Thus, Germany was left with no option but to preemptively invade Poland, thereby halting the alleged Polish plans to invade Germany. It was later discovered that Germany had fabricated the evidence for the alleged Polish saboteurs as a part of the Gleiwitz incident.
World War II period (1939–1945)
Once again, during the course of the even more widespread and lethal World War II, the hope of somehow definitively ending all war (including preemptive war) was seriously discussed. That dialogue ultimately resulted in the re-establishment of the successor organization to the old League, namely the United Nations (UN). As with the League, the primary aim and hope of the new UN was the prevention of all wars (including preemptive wars). Unlike the previous League, the organization had the support of the United States.
In analyzing the many components of World War II, if one might consider as separate individual wars, the various attacks on previously neutral countries, one might consider the attacks against Iran and Norway to have been preemptive wars.
In the case of Norway, the 1940 German invasion of Norway in the 1946 Nuremberg trials the German defense argued that Germany was "compelled to attack Norway by the need to forestall an Allied invasion and that her action was therefore preemptive."  The German defence was referring to Plan R 4 and its predecessors. Norway was vital to Germany as a transport route for iron ore from Sweden, a supply that Britain was determined to stop. One adopted British plan was to go through Norway and occupy cities in Sweden.   An Allied invasion was ordered on March 12, and the Germans intercepted radio traffic setting March 14 as deadline for the preparation.  Peace in Finland interrupted the Allied plans, but Hitler became, rightly, convinced that the Allies would try again, and ordered operation Weseruebung.
The new Allied plans were Wilfred and Plan R 4. The plan was to provoke a German reaction by laying mines in Norwegian waters, and once Germany showed signs of taking action UK troops would occupy Narvik, Trondheim and Bergen and launch a raid on Stavanger to destroy Sola airfield. However, "the mines were not laid until the morning of 8 April, by which time the German ships were advancing up the Norwegian coast."  However The International Military Tribunal at Nuremberg determined that no Allied invasion was imminent, and therefore rejected the German argument that Germany was entitled to attack Norway. 
Pre-September 11, 2001 United Nations period (1945–2001)
Israel incorporates preemptive war in its strategic doctrine because of its lack of strategic depth.  The Six-Day War, which began when Israel launched a successful attack on Egypt on June 5, 1967, has been widely described as a preemptive war     and is, according to the United States State Department, "perhaps the most cited example [of preemption]".  Others have alternatively referred to it as a preventive war. Some have referred to the war as an act of "interceptive self-defense."  According to that view, no single Egyptian step may have qualified as an armed attack, but Egypt’s collective actions made clear that she was bent on armed attack against Israel. One academic has claimed that Israel's attack was not permissible under the Caroline test, as that there was no overwhelming threat to Israel's survival.
Post September 11, 2001 Bush administration period (2002–2008)
The doctrine of preemption gained renewed reputation following the U.S. invasion of Iraq. Bush administration mainly claimed for the necessity to intervene to prevent Saddam Hussein from deploying weapons of mass destruction (WMD) prior to launching an armed attack. At that time, U.S. decision-makers claimed Saddam's WMD might be given to terrorists groups and claimed that security of the nation was at a great risk. Soon the Congress passed its joint resolution in October 2002 authorizing the president to use military force against Saddam Hussein's regime. However, it was later confirmed that no nuclear weapons or biological weapons capability existed and that Bush administration's suspicion was mistaken. Some questioned the true intention of Bush administration for invading Iraq, based on possibility of retaliation on the terrorist attacks which occurred on September 11, 2001. It is still unclear whether the U.S. invasion of Iraq is legally justifiable and, at the same time, whether Iraq’s resistance to the attack is justifiable.
Arguments for preemptive war made during the Bush administration
Sofaer's four elements
- The nature and magnitude of the threat involved;
- The likelihood that the threat will be realized unless preemptive action is taken;
- The availability and exhaustion of alternatives to using force; and
- Whether using preemptive force is consistent with the terms and purposes of the U.N. Charter and other applicable international agreements.
Walzer's three elements
- The existence of an intention to injure;
- The undertaking of military preparations that increase the level of danger; and
- The need to act immediately because of a higher degree of risk.
The counter proliferation self-help paradigm
The proliferation of WMDs by rogue nations gave rise to a certain argument by scholars concerning preemption.  They argued that the threat need not be "imminent" in the classic sense and that the illicit acquisition of these weapons, with their capacity to unleash massive destruction, by rogue states, created the requisite threat to peace and stability as to have justified the use of preemptive force. NATO's Deputy Assistant Secretary General for WMD, Guy Roberts cited the 1962 Cuban Missile Crisis, the 1998 US attack on a Sudanese pharmaceutical plant, (identified by US intelligence to have been a chemical weapons facility) and the 1981 Israeli attack on Iraq's nuclear facility at Osirak as examples of the counter-proliferation self-help paradigm. Regarding the Osirak attack, Roberts noted that at the time, few legal scholars argued in support of the Israeli attack but notes further that, "subsequent events demonstrated the perspicacity of the Israelis, and some scholars have re-visited that attack arguing that it was justified under anticipatory self-defense." Following the U.S. invasion of Iraq in 2003, American forces captured a number of documents detailing conversations that Sadaam Hussein had with his inner sanctum. The archive of documents and recorded meetings confirm that Hussein was indeed aiming to strike at Israel. In a 1982 conversation Hussein stated that, "Once Iraq walks out victorious, there will not be any Israel." Of Israel's anti-Iraqi endeavors he noted, "Technically, they [the Israelis] are right in all of their attempts to harm Iraq."
Post–Bush administration period (2009–present)
Since the departure of the Bush administration, the Obama administration has made no such claims to retain the right to declare a preemptive war, but has adopted and continued many polices of the Bush Doctrine. 
The intention with a preemptive strike is to gain the advantage of initiative and to harm the enemy at a moment of minimal protection, for instance, while vulnerable during transport or mobilization; however the concept of preemptive war can be used to start a war by claiming that the nation would soon be under attack and therefore had to defend itself. The concept is controversial because it can be used as a justification to start a war on questionable grounds.
In his work titled "Rationalist Explanations for War", James Fearon attributes the use of preemptive strikes by rational states to both offensive advantages and commitment problems between states. When a nation possesses a first strike advantage and believes itself to have a high probability of winning a war, then there is a narrower de facto bargaining range between them and an opposing country for peaceful settlements. In extreme cases, if the probability of winning minus the probable costs of war is high enough, then no self-enforcing peaceful outcome exists. In his discussion of preventative war arising from a commitment problem, Fearon builds an infinite-horizon model expected payoffs from period t on are (pt/(l - δ)) - Ca for state A and ((1 -pt)/(l - δ)) - Cb for state B, where Ca and Cb are costs incurred the respective states and δ is the state discount of the future period payoffs. This model shows that a peaceful settlement can be reached at any period that both states prefer, but strategic issues arise when there is no credible third-party guaranteer of the both states committing to a peaceful foreign policy. If there is going to be a shift in the military power between states in the future and no credible restraint is placed on the rising military power not to exploit its future advantage, then it is rational for the state with declining military power to use a preventative attack while it has a higher chance of winning the war. Fearon points out that the declining state attacks not due to fear of a future attack, but because the peace settlement in the future would be worse for it than in the current period. The lack of trust that leads to a declining power's preemptive strike does not stem from uncertainty about intentions of different nations, but rather "the situation, the structure of preferences and opportunities, that gives one party incentive to renege" on its peaceful cooperation and exploit its increased military potential in the future to win a more profitable peace settlement for itself. Thus, Fearon shows that preemptive military action is taken by nations when there is an unfavorable shift in military potential in the future which leads to a shrinking bargain range for a peaceful settlement in the current period, but with no credible commitment by the other party to not exploit their improved military potential in the future.
Article 2, Section 4 of the U.N. Charter is generally considered to be jus cogens (literally: "compelling law", in practice: "higher international law"), and prohibits all UN members from exercising "the threat or use of force against the territorial integrity or political independence of any state". But in the modern framework of the UN Charter, it is the phrase "armed attack occurs" in Article 51 that draws the line between legitimate and illegitimate military force. Some scholars believe it is reasonable to assume that if no armed attack has yet occurred that no automatic justification for preemptive 'self-defense' has yet been made 'legal' under the UN Charter. In order to be justified as an act of self-defense, two conditions must be fulfilled which are widely regarded as necessary for its justification. The first of these is that actor must have believed that the threat is real, as opposed to (merely) perceived. The second condition is that the force used in self-defense must be proportional to the harm which the actor is threatened. When it comes to a situation where an armed attack is considered as a self-defense, it usually narrows realistic options for avoidance by nonviolent means such as negotiation, retreat, or calling upon higher authorities (such as the police or the UN).
- Controversies relating to the Six-Day War
- A Clean Break: A New Strategy for Securing the Realm
- Battleplan (documentary TV series)
- Bush Doctrine
- Caroline affair
- Dwight D. Eisenhower statements on ' preventive war' in WikiQuote
- Jus ad bellum
- War of aggression
- Pre-emptive nuclear strike (counterforce)
- Soviet offensive plans controversy
- Hobbesian trap