Customary international law in the context of "Occupying power"

The use of force by states and inter-governmental organizations in international law is controlled by both customary international law and by treaty law. As a legal matter, the use of force may be justified only in self-defense or when authorized by the United Nations Security Council. The UN Charter reads in article 2(4):

This principle is now considered to be a part of customary international law, and has the effect of banning the use of armed force except for two situations authorized by the UN Charter. Firstly, the Security Council, under powers granted in articles 24 and 25, and Chapter VII of the Charter, may authorize collective action to maintain or enforce international peace and security. Secondly, Article 51 also states that: "Nothing 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." There are also more controversial claims by some states of a right of humanitarian intervention, reprisals and the protection of nationals abroad. Concomitant with Article 51’s right to self-defense it Article 2(4)’s prohibition against “the threat or use of force against the territorial integrity or political independence of any state."

A war crime is a violation of the laws of war that gives rise to individual criminal responsibility for actions by combatants in action, such as intentionally killing civilians or intentionally killing prisoners of war, torture, taking hostages, unnecessarily destroying civilian property, deception by perfidy, wartime sexual violence, pillaging, and for any individual that is part of the command structure who orders any attempt to committing mass killings (including genocide or ethnic cleansing), the granting of no quarter despite surrender, the conscription of children in the military, and flouting the legal distinctions of proportionality and military necessity.

The formal concept of war crimes emerged from countries fighting and the codification of the customary international law that applied to warfare between sovereign states, such as the Lieber Code (1863) of the Union Army in the American Civil War and the Hague Conventions of 1899 and 1907 for international war. In the aftermath of World War II, the war-crime trials of the leaders of the Axis powers established the Nuremberg principles, such as that international criminal law defines what is a war crime. In 1899, the Geneva Conventions legally defined new war crimes and established that states could exercise universal jurisdiction over war criminals. In the late 20th century and early 21st century, international courts extrapolated and defined additional categories of war crimes applicable to a civil war.

Military occupation, also called belligerent occupation or simply occupation, is temporary hostile control exerted by a ruling power's military apparatus over a sovereign territory that is outside of the legal boundaries of that ruling power's own sovereign territory. The controlled territory is called occupied territory, and the ruling power is called the occupant. Occupation's intended temporary nature distinguishes it from annexation and colonialism. The occupant often establishes military rule to facilitate administration of the occupied territory, though this is not a necessary characteristic of occupation.

The rules of occupation are delineated in various international agreements—primarily the Hague Convention of 1907, the Geneva Conventions, and also by long-established state practice. The relevant international conventions, the International Committee of the Red Cross, and various treaties by military scholars provide guidelines on topics concerning the rights and duties of the occupying power, the protection of civilians, the treatment of prisoners of war, the coordination of relief efforts, the issuance of travel documents, the property rights of the populace, the handling of cultural and art objects, the management of refugees, and other concerns that are highest in importance both before and after the cessation of hostilities during an armed conflict. A country that engages in a military occupation and violates internationally agreed-upon norms runs the risk of censure, criticism, or condemnation. In the contemporary era, the laws of occupation have largely become a part of customary international law, and form a part of the law of war.

Crimes against humanity are certain crimes committed as part of a large-scale attack against civilians. Unlike war crimes, crimes against humanity can be committed during both peace and war and against a state's own nationals as well as foreign nationals. Together with war crimes, genocide, and the crime of aggression, crimes against humanity are one of the core crimes of international criminal law and, like other crimes against international law, have no temporal or jurisdictional limitations on prosecution (where universal jurisdiction is recognized).

The first prosecution for crimes against humanity took place during the Nuremberg trials and Subsequent Nuremberg trials against defeated leaders of Nazi Germany and collaborators. Crimes against humanity have been prosecuted by other international courts (such as the International Criminal Tribunal for the former Yugoslavia, the International Criminal Tribunal for Rwanda, the Special Court for Sierra Leone, the Extraordinary Chambers in the Courts of Cambodia, and the International Criminal Court) as well as by domestic courts. The law of crimes against humanity has primarily been developed as a result of the evolution of customary international law. Crimes against humanity are not codified in an international convention, so an international effort to establish such a treaty, led by the Crimes Against Humanity Initiative, has been underway since 2008.

International human rights law (IHRL) is the body of international law designed to promote human rights on social, regional, and domestic levels. As a form of international law, international human rights law is primarily made up of treaties, agreements between sovereign states intended to have binding legal effect between the parties that have agreed to them; and customary international law. Other international human rights instruments, while not legally binding, contribute to the implementation, understanding and development of international human rights law and have been recognized as a source of political obligation.

International human rights law, which governs the conduct of a state towards its people in peacetime is traditionally seen as distinct from international humanitarian law which governs the conduct of states and non-state armed groups during conflict, although the two branches of law are complementary and in some ways overlap.

The Geneva Conventions is a term used to collectively refer to the four Geneva Conventions of 1949 along with their two optional protocols (Additional Protocols I and II), which form the core of international humanitarian law and establish international legal standards for the treatment of non-combatants in war. Negotiated in the aftermath of the World War II, the Geneva Conventions of 1949 consist of four agreements, which replaced three earlier humanitarian conventions of 1906 and 1929 and added two new conventions. The First Geneva Convention addresses the treatment of sick and wounded field soldiers, Second Geneva Convention addresses the treatment of sick and wounded sailors, Third Geneva Convention addresses the treatment of prisoners or war, and Fourth Geneva Convention addresses the treatment of civilians during armed conflict.

Aside from the UN Charter, the Geneva Conventions are among the most widely ratified international agreements. The 1949 conventions were ratified, in their entirety or with reservations, by 196 countries. Much of the international humanitarian law contained in the Geneva Conventions has become customary international law, and therefore would apply to all states and non-state armed groups, even those that are not a party to the Geneva Conventions. The legal obligations created under the Geneva Conventions are somewhat different from other international treaties in that a breach of the Geneva Conventions by one party does not furnish a directly affected party with the option to cease complying with its treaty obligations.

The law of war is the part of international law that regulates the conditions for initiating war (jus ad bellum) and the conduct of hostilities (jus in bello). Laws of war define sovereignty and nationhood, states and territories, occupation, and other critical terms of law. Among other issues, the modern laws of war address the declarations of war; acceptance of surrender and the treatment of prisoners of war; the principles of distinction, as well as military necessity and proportionality; and the prohibition of certain weapons that cause unnecessary or excessive suffering.

The law of war is considered distinct from other bodies of law—such as the domestic law of a particular belligerent to a conflict—which may provide additional legal limits to the conduct or justification of war. The term law of war, or jus in bello, overlaps to some degree with opinio juris (also referred to as customary law), but refers to a very specific body of international law composed of customary and treaty-based law.

Pacta sunt servanda ("agreements must be kept") is a brocard and a fundamental principle of law which holds that treaties or contracts are binding upon the parties that entered into the treaty or contract. It is customary international law. According to Hans Wehberg, a professor of international law, "few rules for the ordering of Society have such a deep moral and religious influence" as this principle.

In its most common sense, the principle refers to private contracts and prescribes that the provisions, i.e. clauses, of a contract are law between the parties to the contract, and therefore implies that neglect of their respective obligations is a violation of the contract. The first known expression of the brocard is in the writings of the canonist Cardinal Hostiensis from the 13th century AD, which were published in the 16th.