Preemptive war occurs when a state that is about to be attacked decides to strike first at its enemy and thus disrupt the impending attack. Unlike preventive war, in which a state strikes a potential enemy even during a time of relative peace (an action that is usually inadmissible under international law), a preemptive war takes place when a state is under the direct threat of imminent hostilities. Preemptive war is therefore a form of self-defense, and is generally accepted as a legitimate use of force under international law.
A modern example of preemptive war occurred in 1967 at the start of the so-called Six-Day-War between Israel and its Arab neighbors. On May 15, 1967, Egypt and Syria, after weeks of intensifying tensions with Israel, concentrated large numbers of military forces on Israel's border. The leader of Egypt, Gamal Abdel Nasser, proclaimed his readiness to make war against Israel, and Egyptian forces were later joined by units from Jordan, Syria, Iraq, and other Arab states. Nasser also demanded that the United Nations peacekeeping forces standing between Israel and Egypt leave the area, and on May 22 he blockaded the Straits of Tiran, closing off navigation to Israeli vessels. These were all evident signs of an impending attack against Israel by a massive force of Arab troops.
In response to this imminent threat, Israel launched a preemptive strike against the Arab forces on June 5, 1967. The Israeli preemption began with a surprise attack against the Egyptian air force that was designed to wipe out as many Egyptian aircraft as possible while they were still on the ground. This campaign was highly successful. The Israelis scored several major military victories by destroying enemy forces before they were ready to engage in hostilities, and within five days Israeli military units were approaching both the Egyptian and Syrian capitals. Egypt, Jordan, and Syria all sought to make peace, and lost significant territory to the Israelis as part of the postwar settlement. Israel's actions in the Six-Day War have since stood as a classic example of a successful preemptive war.
The concept of preemption was of particular concern during the Cold War (1946–1991) between the United States and the Soviet Union because of the nature of the ballistic missile weapons that both nations possessed (and continue to possess). Nuclear missiles are ideal weapons for a surprise attack; a missile launched from Europe or Asia would reach the continental United States in less than thirty minutes, and even sooner if launched from a submarine in nearby waters. American missiles could reach Soviet territory in an equally short time. This capability had important implications during a time of crisis: if both the U.S. and USSR became convinced that a war was imminent and inevitable, then both nations would have a strong incentive to try to launch their missiles first and thus preempt the enemy, resulting in an accidental war that neither side may have wanted. Since the fall of the Soviet regime in 1991, the United States and the new Russian Federation have agreed not to target their missiles at each other so as to lessen this kind of a risk.
As more states develop nuclear missile forces in the future, however, the problem of preemption will continue to be a concern to military strategists and policymakers in many countries.
Thomas M. Nichols