Pre-emptive strike or pre-emptive war? Basic requirements?

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. This is the same as preemptive war but is often confused with the term preventive war. While the latter is generally considered to violate international law, and to fall short of the requirements of a just war, preemptive wars are more often argued to be justified or justifiable (although international law categorically rejects a preemptive strike or preemptive war). My question is: Name at least 6 basic requirements to succeed in such an awesome undertaking