A war in which the attacked country poses no direct or imminent threat to the attacker. To be differentiated from preemptive war, in which the attacker is in clear danger of being attacked.
The US is an old hand at staging preventive wars. Through clever use of propaganda to infuse nationalism and support for its wars among the population it is able to attack and invade sovereign countries under the guise of "liberation" or "preemptive attack".
See also War with Iraq.
I'll go and punch a blind person in a wheelchair on the street and call it a preventive attack. He was clearly going for my wallet!