Proportionality is a fundamental component of the law on the use of force and the law of armed conflict—the jus ad bellum and the jus in bello. In the former, it refers to a belligerent’s response to a grievance and, in the latter, to the balance to be struck between the achievement of a military goal and the cost in terms of lives. The legitimate resort to force under the United Nations system is regarded by most commentators as restricted to the use of force in self-defense under Article 51 and collective security action under chapter VII of the UN Charter. The resort to force in both these situations is limited by the customary law requirement that it be proportionate to the unlawful aggression that gave rise to the right. In the law of armed conflict, the notion of proportionality is based on the fundamental principle that belligerents do not enjoy an unlimited choice of means to inflict damage on the enemy. Since the entry into force of Protocol I to the Geneva Conventions of 12 August 1949, and Relating to the Protection of Victims of International Armed Conflicts, proportionality has been both a conventional and a customary principle of the law of armed conflict.