The Law of Armed Conflict
How international law regulates when states may use force and how military operations must be conducted.
When Can States Use Force?
The law governing the resort to force is called jus ad bellum. The UN Charter establishes the fundamental rule: the use of force against another state is prohibited (Article 2(4)). There are only two exceptions. First, self-defense under Article 51: a state may use force if it is the victim of an armed attack, and only until the Security Council takes necessary measures. Second, authorization by the Security Council under Chapter VII, which has been used to authorize interventions from Korea (1950) to Libya (2011).
These rules sound clear but are endlessly contested. Does self-defense include preemptive strikes against imminent threats? The US and Israel say yes; most states and scholars say it requires an actual armed attack. Does humanitarian intervention without Security Council authorization violate Article 2(4)? NATO's 1999 intervention in Kosovo was widely considered illegal but legitimate. The Responsibility to Protect (R2P) doctrine attempts to bridge this gap but has no firm legal standing.
