There is no reason to think that only States can wage aggressive wars. In contemporary times, we have seen non-State actors, including terrorist groups, wage war against States and against other non-State actors. I will adopt an amended version of a definition offered by Andrew Valls and define terrorism as violence committed by State or non-State actors directed against civilians or their property for political purposes. In Just War theory, wars by non-State actors were not generally contemplated because the legitimate authority to wage war resided only in States. There is an interesting exception, which I will explore, concerning piracy as a form of aggression by non-State actors. Throughout, I will argue that the jus ad bellum principles we have been discussing in this book apply to some terrorist groups and that when the leaders of such groups wage aggressive war they should be confronted by international legal institutions. I will also argue that when terrorists are confronted and prosecuted they should be afforded the same rights as State leaders when the latter are similarly confronted and prosecuted.

The strategy I adopt, of allowing that some terrorist groups can wage war and that terrorist leaders can be prosecuted for waging aggressive war as long as such prosecutions are subject to the rule of law, recognizes the reality of the contemporary situation where there are multiple types of actors on the world stage.