The basic requirement of jus ad bellum is possessing just cause. The clearest example of just cause would be the right of self-defense: if a nation is attacked it has the right to defend itself. By extension, this also means the right to defend other nations from aggression to meet treaty obligations or under the direction of an international regime such as the United Nations. By further extrapolation, the right of self-defense allows for preemptive action if an attack is imminent.5 This does not mean that a preventative war to forestall an attack at sometime in the indeterminate future is necessarily just. Even in the clearest cases of aggression, however, the principle of last resort requires that all peaceful means of resolving disputes
