A preemptive war is a war that is commenced in an attempt to repel or defeat a perceived offensive or invasion, or to gain a strategic advantage in an impending (allegedly unavoidable) war before that threat materializes. It is a war which 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 an enemy, when an attack by that party is not imminent or known to be planned, while a preemptive war is launched in anticipation of immediate enemy aggression. 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 (see 'Legality' below), 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 1525, Hugo Grotius characterized a state's right of self-defense to include the right to forcibly forestall an attack. 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, U.S. Secretary of State Daniel Webster pointed out that the necessity for forcible reaction must be "instant, overwhelming, leaving no choice of means, and no moment for deliberation." This formulation is part of the Caroline test, which "is broadly cited as enshrining the appropriate customary law standard".
World War I period (1914 - 1918) 
During the course of the destructive and costly World War I, for the first time in history, the concept of the "War to end all wars," began to be seriously considered. As a further expression of this hope, upon the conclusion of the war, the League of Nations (LON) was formed. The primary aim of this organization 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 this agreement, with the notable exception of the United States.
League of Nations period (1919 - 1939) 
During the 1920s, the LON 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, starting with the Mukden Incident. 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 certain bridge near Mukden, China. Therefore clearly the Chinese were the aggressors, and the Japanese were merely 'defending themselves'. A predominance of evidence has since indicated that the bridge had in fact most probably been blown up by Japanese operatives.
In 1933 the impotency of the LON became more pronounced when notices were provided by Japan and Germany that they would be terminating their memberships in the League of Nations. Italy shortly followed suit and exited the League in 1937. Soon also Italy and Germany began engaging in militaristic campaigns designed to either enlarge their borders or to expand their sphere of military control, and the League of Nations was shown to be powerless to stop them. This perceived impotency of the League of Nations 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. This dialogue ultimately resulted in the re-establishment of the successor organization to the old LON, namely the United Nations (UN). As with the LON, the primary aim and hope of the new UN was the prevention of all wars (including preemptive wars). Unlike the previous LON, 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, then 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) 
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 this view, though no single Egyptian step may have qualified as an armed attack, 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, arguing 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 faced the threat of WMD was imminent and believed 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, later it has been confirmed that no WMD was found and that Bush administration’s suspicion was mistaken. Some questions 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, the Iraqis’ resistance of 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 nations, 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 to 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 abused and used as a justification to start a war on questionable grounds.
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 U.N. 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.  From this 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) perceive. 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). 
See also 
- 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
- First strike
- Soviet offensive plans controversy
- Beres, Louis Rene (1991-1992), On Assassination as Anticipatory Self-Defense: The Case of Israel 20, Hofstra L. Rev., p. 321
- Shue, Henry and Rhodin, David (2007). Preemption: Military Action and Moral Justification. Oxford University Press. p. 116. ISBN 978-0-19-923313-7
- Shue and Rodin 2007, p. 118.
- "The Implications of Preemptive and Preventive War Doctrines: a Reconsideration". 2007. Retrieved 2010-12-02. A US Army sponsored discussion of various justifications for preemptive, preventive and 'precautionary' war.
- "Adoption of Policy of Pre-emption Could Result in Proliferation of Uniliteral, Lawless Use of Force: By Kofi Annan". 2003. Retrieved 2010-12-02. Kofi Annan discusses his unwillingness to accept proposed new changes in UN policy towards the use of preemptive force, and why.
- Beres, Louis R. (1991), Permissibility of State-Sponsored Assassination during Peace and War, The 5, Temp. Int'l & Comp. L.J., p. 231
- Duffy, Helen (2005). The 'War on Terror' and the Framework of International Law. Cambridge University Press. p. 157.ISBN 978-0521547352
- "United States History: Woodrow Wilson". 2010. Retrieved 1010-11-30. Discussion of Woodrow Wilson's desire to make World War I the 'War to End All Wars.'
- "Eleanor Roosevelt National Historic Site: League of Nations article". 2010. Retrieved 2010-11-30. Article summarizing the primary objective of the League of Nations.
- "Mukden Incident and Manchukuo: C. Peter Chen". 2010. Retrieved 2010-11-30. Details of the Mukden Incident.
- "League of Nations Timeline". 2006. Retrieved 2010-11-30. A timeline of all major LON events.
- "Factmonster Encyclopedia— League of Nations: Successes and Failures". 2010. Retrieved 2010-11-30. Description of the demise of the League of Nations.
- Myres Smith McDougal, Florentino P. Feliciano, "The international law of war: transnational coercion and world public order" p 211, 212
- "COMMAND DECISIONS", CENTER OF MILITARY HISTORY DEPARTMENT OF THE ARMY WASHINGTON, D.C., 2000. URL p. 59 "The British plan which was adopted was more modest. While ostensibly intended to bring Allied troops to the Finnish front, it laid its main emphasis on operations in northern Norway and Sweden. The main striking force was to land at Narvik and advance along the railroad to its eastern terminus at Lulea, occupying Kiruna and Gallivare along the way. By late April two Allied brigades were to be established along that line."
- "COMMAND DECISIONS", CENTER OF MILITARY HISTORY DEPARTMENT OF THE ARMY WASHINGTON, D.C., 2000. URL p. 66, 67 "The British held back two divisions from France, intending to put them into the field in Norway, and planned to expand their force eventually to 100,000 men. The French intended to commit about 50,000. The British and French staffs agreed that the latter half of March would be the best time for going into Norway;"
- "COMMAND DECISIONS", CENTER OF MILITARY HISTORY DEPARTMENT OF THE ARMY WASHINGTON, D.C., 2000. URL p.67,68 "The objectives were to take Narvik, the railroad, and the Swedish ore fields;" "an intercepted radio message setting 14 March as the deadline for preparation of transport groups indicated that the Allied operation was getting under way. But another message, intercepted on the 15th, ordering the submarines to disperse revealed that the peace [in Finland] had disrupted the Allied plan."
- "COMMAND DECISIONS", CENTER OF MILITARY HISTORY DEPARTMENT OF THE ARMY WASHINGTON, D.C., 2000. URL p. 68
- Jus ad Bellum: Law Regulating Resort to Force
- The Six Day War is, "A classic example of preemptive war." Henry Shue, David Rodin Preemption: military action and moral justification
- "Classic examples of preemptive wars include the July Crisis of 1914 and the Six Day War of 1967 in which Israel preemptively attacked Egypt…" Karl P. Mueller Striking first: preemptive and preventive attack in U.S. national security
- “The Six Day War between Israel and alliance of Egypt, Syria, Jordan and Iraq was an example of preemption.” And, “It exemplifies preemption.” Charles W. Kegley, Gregory A. Raymond The Global Future: A Brief Introduction to World Politics
- "Preemptive attack is morally justified when three conditions are fulfilled: 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. Since these conditions were met in Israel’s Six Day War, Israel’s preemptive attack on Egypt on June 5, 1967 was a legitimate act of self-defense.” Mark R. Amstutz International ethics: concepts, theories, and cases in global politics
- “The United States has often walked a fine line between preemption and prevention. In fact there have only been a handful of clear-cut cases of military preemption by any states in the last 200 years. (Israeli preemption in the Six Day War of 1967 is perhaps the most cited example)” U.S. National Security Strategy: a New Era U.S. Department of State (2002)
- Choice or Necessity (New York Times, May 8, 2009)
- Distein, Yoram, War, aggression and self-defense p. 192, Cambridge University Press (2005)
- "The closest case that might have, but is now regarded as not having met the Caroline test, was Israel's first strike against Egypt in 1967. Few regarded it as a good example of a permissible anticipatory attack under the Caroline test, especially after it became clear following the attack that there was no overwhelming threat that justified the attack to ensure Israel's survival. Gathii, James Thuo. Assessing Claims of a New Doctrine of Preemptive War Under the Doctrine of Sources. Osgoode Hall Law Journal, Vol. 43, No. 1, pp. 1-34, 2005.
- George, and Jens Ohlin. Defending humanity. New York: Oxford University Press, 2008. Print.
- Abraham D. Sofaer. On the Necessity of Pre-emption. European Journal of International Law, Vol. 14 No. 2, 2003, p.220 
- Mark R. Amstutz, International ethics: concepts, theories, and cases in global politics
- Col Guy Roberts, USMC (Ret) 27 Denv. J. Int'l L. & Pol'y 483
- Steven C. Welsh, Preemptive War and International Law Center for Defense Information, 5 December 2003
- Kacerauskis p. 84-85
- Roberts, n. 528-536
- Roberts n. 530-532
- Gordon, Michael, R. (25 October 2011). "Papers From Iraqi Archive Reveal Conspiratorial Mind-Set of Hussein". New York Times.
- Krauthammer, Charles (23 May 2011). "Obama adopts the Bush Doctrine". Chicago Tribune.
- David, and Henry Shue. Preemption : Military Action and Moral Justification. New York: Oxford University Press, 2007. Print.
- Preemptive warfare: A viable strategic option
- Can a member of The United Nations unilaterally decide to use preemptive force against another state without violating the UN Charter? Vytautas Kacerauskis, International Journal of Baltic Law Volume 2, No. 1 (January, 2005) ISSN 1648-9349
- Washington Times
- ThoughtCast speaks with Alan Dershowitz about his book "Preemption: A Knife that Cuts Both Ways"...
- The American Strategy of Preemptive War and International Law
- The Myth of Preemptive Self-Defense Mary Ellen O’Connell Professor of Law
- Pre-emptive Self-Defence, International Law and US Policy Chris Richter
- U.S. National Security Strategy: a New Era U.S. Department of State