The body of international law that governs the use of force between states is called jus ad bellum. Jus ad bellum determines when one state may lawfully use force against another. It is sometimes called jus contra bellum to reflect that it is a legal regime based on the general prohibition of the use of force between states.
Under customary international law Customary international law consists of unwritten rules that come from a general practice accepted as law. It exists independently from treaty law and is binding on all states and the Charter of the United Nations Pursuant to Article 2 (4) of the Charter of the United Nations, '[a]ll Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.', the use of force is prohibited with two generally accepted exceptions. First, the United Nations Security Council may authorize the use force to maintain and restore peace and security. Under the collective security system set up by Chapter VII of the Charter of the United Nations, the Security Council is to take measures in case of a threat to the peace, breach of the peace or act of aggression. Such measures include the authorization of military action. Second, states may use force when acting in self-defence against an armed attack. According to Article 51 of the United Nations Charter, ‘[n]othing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.' With the exception of the purely procedural provision, this provision expresses customary international law. Not every use of force amounts to an armed attack. An armed action must present a certain degree of gravity to entail the right to self-defence. Although not conceived for this purpose, Article 3 of United Nations General Assembly resolution 3314 (XXIX) on the Definition of Aggression provides guidance for evaluating whether armed action amounts to an armed attack. The right to self-defence may be exercised individually or collectively. For self-defence to be lawful, the use of force in response to an armed attack must be both necessary and proportionate. It remains controversial whether the right to self-defence encompasses the right to self-defence against an armed attack by a non-state actor or a right to pre-emptive self-defence. Similarly, other purported exceptions such as humanitarian intervention or the use of force under the responsibility to protect doctrine are contested. The concept of humanitarian intervention or responsibility to protect purportedly allows states to use of force in order to prevent gross human rights violations in another country without the authorization of the United Nations Security Council.
Jus ad bellum and international humanitarian law
International humanitarian law operates independently from questions surrounding the legality of the use of force between states. Regardless of the legality of the use force or more generally the reasons given for resorting to force, the obligations under international humanitarian law remain the same for all parties to a conflict. In other words, the application of humanitarian law does not involve a judgment on the legality or legitimacy of the resort to force. For example, a state claiming to act in self-defence against foreign aggression has exactly the same obligations under international humanitarian law as the purported aggressor. Any other approach would led to controversy and undermine international humanitarian law as each party would argue to be the victim of aggression.
Similarly, international humanitarian law does not involve a judgment on the legality or legitimacy of use of force by armed groups. Taking up arms against the state is not a violation of international humanitarian law. In contrast, domestic law usually prohibits taking up arms against the state. Therefore, members of armed groups may face prosecution and punishment under domestic law for taking up arms, including for acts that are lawful under international humanitarian law.
Interventions short of the use of force
Coercive acts that do not amount to a use of force are prohibited under the principle of non-intervention. The Declaration on Principles of International Law concerning Friendly Relations and Cooperation among States in accordance with the Charter of the United Nations states that ‘[n]o State may use or encourage the use of economic political or any other type of measures to coerce another State in order to obtain from it the subordination of the exercise of its sovereign rights and to secure from it advantages of any kind. Also, no State shall organize, assist, foment, finance, incite or tolerate subversive, terrorist or armed activities directed towards the violent overthrow of the regime of another State, or interfere in civil strife in another State.’ See General Assembly Resolution 2625 (XXV), UN doc A/RES/25/2625, 1970. Direct or indirect support of a foreign state to an armed group opposing the territorial government in the form of funds, weapons, or training constitutes a breach of the principle of non-intervention. In the case concerning the support of the United States to the Nicaraguan Contras, the International Court of Justice ‘[w]as unable to consider that […] the provision of arms to the opposition in another State constituted an armed attack on that State'. At the same time, the Court ‘[f]ound that the support given by the United States […] to the military and paramilitary activities of the contras in Nicaragua, by financial support, training, supply of weapons, intelligence and logistic support, constituted a clear breach of the principle of non-intervention. Case Concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America), Judgment (Merits), 27 June 1986, §230 and §242.