Complex sentence: The difference is that a preventive war is launched to destroy the potential threat of the targeted party, when an attack by that party is not imminent or known to be planned, while a preemptive war is launched in anticipation of immediate aggression by another party.
Split into simpler sentences: The difference is that a preventive war is launched to destroy the potential threat of the targeted party, when an attack by that party is not imminent or known to be planned. A preemptive war is launched in anticipation of immediate aggression by another party.