Just cause is the single most important factor governing the decision to make war under customary international law. Interestingly enough, customary international law reached its pinnacle by the late 18th century, when the United States adopted its Constitution. Consequently, the basic criteria and distinctions of customary law, particularly those pertaining to just cause, were influential in the Constitution's allocation of war powers. Just cause is also the central element in the centuries-old just war doctrine of Christian thought. Indeed, customary international law adopted the concept of just cause nearly whole cloth from just war doctrine. Accordingly, just cause is the unifying thread that runs through customary international law, the U.S. Constitution, and the just war doctrine.
Jeffrey C. Tuomala, Just Cause: The Thread that Runs So True, 13 Penn St. Int'l L. Rev. 1 (1994).