The following is the March 16, 2022, Congressional Research Service Legal Sidebar, The Law of War and the Russian Invasion of Ukraine.
From the report
In the days after Russia’s invasion of Ukraine on February 24, 2022, many countries condemned the action as a violation of international law governing when countries may use force against one another. Since then, several observers, including the U.S. Secretary of State and other foreign government officials, have cited evidence that the Russian military has targeted civilians, struck protected sites, and taken other actions that violate international law regulating the conduct of war. This Legal Sidebar provides a brief introduction to the international legal framework governing the use of force in the invasion of Ukraine and concludes with a discussion of avenues for accountability and options for Congress.
The law of war generally refers to the portion of international law that regulates the inception of use of force, the conduct of hostilities, and the protection of war victims, among other things. The term is often used interchangeably with the law of armed conflict and international humanitarian law. There are two major categories under the law of war umbrella: jus ad bellum (legal rules governing when a country can resort to use of force) and jus in bello (law governing conduct during the use of force). While they can be interrelated, jus ad bellum and jus in bello generally operate independently such that compliance with one category is required regardless of compliance with the other. For example, a state that is a victim of a jus ad bellum violation because it is attacked without a lawful basis must still comply with jus in bello when conducting military operations to defend itself.
Jus ad Bellum: Deconstructing the Justifications for War in Ukraine
The starting point to analyze most aspects of jus ad bellum is the U.N. Charter. Article 2(4) prohibits member-states from using or threatening to use force against one another, but there are exceptions. Article 51 preserves member-states’ right to act in either individual or collective self-defense when an armed attack occurs, and Chapter VII of the charter permits the U.N. Security Council to authorize military actions necessary to maintain or restore international peace and security. A state can also consent to the use of force in its territory.
Customary international law (described in this CRS Report) also informs jus ad bellum. Customary international law requires that the use of force be proportionate to the justification for military action and necessary because no other reasonable means of redress are available.
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