- See also: Preventive attack
Preemptive attack is a military doctrine in which an actor uses military force on an opponent that it believes it is about to attack the actor. It is a spoiling attack to disrupt the preparations for offensive warfare, and implies a response to an immediate danger. Preemption is considered an action of self-defense within the scope of Article 51 of the United Nations Charter.
It contrast, a preventive attack is intended to spoil a capability, not being readied for immediate attack, which presents a long-term threat. A preemptive attack may or may not be intended to completely stop a potential war. Patrons of a country receiving such an attack are wise to move cautiously until it is determined if the attack is part of a larger offensive campaign.
Charles Western quotes Nye's definition that
A premptive war occurs when statesmen believe merely that war is better now than later A preemptive strike occurs when war is imminent. 
For an attack to be preemptive, it must comply with three criteria, first established by Daniel Webster in the Caroline case of 1942. The Caroline was a U.S. ship in Canadian waters, sunk by the Royal Navy because it was believed to be supporting rebels. The criteria, to give the act legitimacy are:
- imminence: There was an immediate and plausible threat. Self-defense does not require taking the first blow.
- proportionality: the amount of force must be appropriate to neutralize the threat, but not more.
- necessity or "window of opportunity": This must be the last opportunity to carry out the act of preemption. While this was fairly straightforward in cases involving overt military forces, it is much more difficult with respect to terrorists, because the only opportunity to attack them may be while they are traveling to the target area, but not in it.
The National Security Strategy of the United States, as stated by the George W. Bush Administration, does consider preventive war as one of many grand strategic options against terrorism and weapons of mass destruction. 
The term suggests that the opponent has immediate hostile intentions to the actor. Examples would be the Israeli attack on Egyptian and Syrian airfields where aircraft were massing for the 1967 Arab-Israeli War.
While prevention is usually assumed to start from a condition of relevant peace, the term has been used to describe attacks against operational military facilities, of an overt opponent, such as the Allied attacks, as part of Operation CROSSBOW, against launching ramps and actual V-1 cruise missiles.  In contrast, the major Briticsh bombing raids against the German long-range guided missile development center at Peenemunde were preventive. Both were in the context of an ongoing war, and indeed a larger operational plan against missile threat.
In the contemporary context, the term is accepted for an attack on a terrorist group about to stage an operation, or a weapon of mass destruction actively being prepared for use. The Iraq War, however, is usually described as preventive rather than preemptive, although the George W. Bush Administration asked Congress for an authorization for the use of military force, in part, as a security measure against terror and WMD. It was not suggested Iraq was likely to use, in the immediate future, terrorism or WMD against the United States.
- Grimmett, Richard F. (2003), U.S. Use of Preemptive Military Force, Congressional Research Service, Library of Congress, CRS report for Congress, RS21311
- , Chapter VII: Action with Respect to Threat to the Peace, Breaches of the Peace, and Acts of Aggression, Article 51, United Nations Charter, United Nations
- Bar-Noi, Uri, The Soviet Union And The Six-Day War: Revelations From The Polish Archives, Cold War International History Project, CWIHP e-Dossier No. 8
- Western, Charles A. (June 17, 2005), Abroad, in Search of Monsters to Destroy: The United States and the Future of Preemption, U.S. Army Command and General Staff College, pp. 9-10
- Nye, Joseph S. (2003), Understanding international conflicts: An introduction to theory and history, Longman, p. 157
- Western, pp. 44-46
- George W. Bush (2006), III. Strengthen Alliances to Defeat Global Terrorism and Work to Prevent Attacks Against Us and Our Friends. C: The Way Ahead, National Security Strategy of the United States
- Reiter, Dan (April 2006), Preventive War and its Alternatives: the Lessons of History, Strategic Studies Institute, U.S. Army War College, pp. 2-11
- Miller, Donald L. (2006), Masters of the Air: America's Bomber Boys Who Fought the Air War Against Nazi Germany, Simon and Schuster, ISBN 0743235444, pp.297-298
- "Peenemunde - 1943", Globalsecurity