Preemptive war (or a preemptive strike) is waged in an attempt to repel or defeat a perceived inevitable offensive or invasion, or to gain a strategic advantage in an impending (allegedly unavoidable) war before that threat materializes. Preemptive war is often confused with the term preventive war.
In a classical approach, self-defense is restricted to a response to an armed attack, in accordance with Article 51 of the United Nations Charter. However, following the practice of states an imminent threat emanating from neighbouring state or disturbances by enemy forces from foreign territory may justify preemptive attacks. Legal experts refer to the Caroline affair of 1837 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."
The scholar Abraham D. Sofaer identifies four key elements for justification of preemption:
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.
The Soviet Union's aerial attack on Finland on June 25, 1941, as an answer to the German attack on Russia of June 22 (Operation Barbarossa leading to the Great Patriotic War), can be argued to be an example of such a preemptive attack, although the bombing of residential districts has to be attributed a psychological aim rather than a tactical. Finland's army was mobilized and prepared for both defence and attack, its government had declared its intention to remain outside of the war, and its parliament was assembled to confirm the status of nonbelligerence when attacked.
The Kosovo War was used as an example by British Prime Minister Tony Blair in his first description of his Doctrine of International Community, made in his speech to the Economic Club of Chicago, Illinois on April 24, 1999.
Also, The Bush Doctrine, first explained in the president's commencement speech to the graduating class of the U.S. Military Academy on June 1, 2002 in West Point, New York states that it is the policy of the United States that "preemptive war" or even preventive war may be waged in appropriate circumstances.
There is some question as to the legality of this doctrine under international law. Article 2, Section 4 of the U.N. Charter is generally considered to be 'jus cogens', or a peremptory norm which cannot be violated. It bars the threat or use of force against any state in the absence of an acute and imminent actual threat. At the same time, however, Article 51 clearly permits self defense. The tension between these two principles is evident in the doctrine of preemptive war, which claims to be defensive, yet does not come in response to an attack.