Contemporary Jus Ad Bellum on Use of Force in Self-Defense by States Against Non-State Terrorist Groups -- Limitations, Evolutions and Alternatives

International law is challenged to ensure the legal and legitimate use of force by states against nonstate terrorist groups. Such groups evade easy classification as criminals or armed forces. Their organization, methods, and targets are simultaneously local in application, but global in impact. They attack the foundations of state identity and legitimacy, including the monopoly on use of force, but are not state actors. Nonstate actor threats are not unprecedented, but states and international law have never had to contend with nonstate actors possessing global reach and force, resources, and influence on par with some states, which creates a dilemma for contemporary states and jus ad bellum -- to adhere to principles of international law and customs of legitimate state action, or to ensure the security of the states citizens. The predicament is incompatible with the notion that, in the modern era, the use of force by states is to be limited, governed, and made legitimate by the rule of law. State use of force without legal authorization creates severe issues of legitimacy, with politically and socially destabilizing effects. This paper examines these issues in full and identifies key trends and potential avenues for legal reform.