What is the difference between preemptive and preventive military action?

"Preemption is the taking of military action against a target when there is incontrovertible evidence that the target is about to initiate a military attack. Prevention is the taking of military action against a target when it is believed that an attack by the target, while not imminent, is inevitable, and when delay in attacking would involve greater risk."

"Preemptive attacks are based on the belief that the adversary is about to attack, and that striking first will be better than allowing the enemy to do so. Preemption may be attractive because it promises to make the difference between victory and defeat, or merely because it will make the ensuing conflict less damaging than it would be if the enemy struck first. Preemptive attacks are quite rare, though the possibility of preemption was a central concern of nuclear strategists during the Cold War; the archetypical example is Israel’s attack against Egypt in 1967 that began the Six-Day War.

Preventive attacks are launched in response to less immediate threats. Preventive attack is motivated not by the desire to strike first rather than second, but by the desire to fight sooner rather than later. Usually this is because the balance of military capabilities is expected to shift in the enemy’s favor, due to differential rates of growth or armament, or the prospect that the opponent will acquire or develop a powerful new offensive or defensive capability. Israel’s 1981 raid on the Osirak nuclear facility was a classic preventive attack, as was Operation Iraqi Freedom, the U.S.-led invasion of Iraq in 2003.

Preemptive and preventive attacks have important differences; in addition to those already noted, international law holds that truly preemptive attacks are an acceptable use of force in self-defense, while preventive attacks usually are not. However, they are driven by similar logic, and since it is often useful to talk about both at the same time, the authors use the term anticipatory attack to refer to the broader category that includes both types of strategies. Anticipatory attack can be viewed as a continuum ranging from purely preemptive to purely preventive actions: All of them are offensive strategies carried out for defensive reasons, based on the belief that otherwise an enemy attack is (or may be) inevitable, and it would be better to fight on one’s own terms. Preemptive and preventive attacks are distinct from 'operational preemption,' taking military actions within an ongoing conflict that are intended to reduce the enemy’s capabilities or to achieve other effects by acting before the enemy launches an attack or takes some other undesirable action, such as deploying or dispersing its forces. Anticipatory attacks often involve operational preemption, but need not do so, and operational preemption may occur in any sort of conflict."