The Law of War and the Russian Invasion of Ukraine

"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.

Terminology

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..."
Law of War and Russia 

U.S. Department of Defense. Law of War Manual

"The law of war is part of who we are. George Washington, as Commander in Chief of the Continental Army, agreed with his British adversary that the Revolutionary War would be “carried on agreeable to the rules which humanity formed” and “to prevent or punish every breach of the rules of war within the sphere of our respective commands.” During the Civil War, President Lincoln approved a set of “Instructions for the Government of the Armies of the United States in the Field,” which inspired other countries to adopt similar codes for their armed forces, and which served as a template for international codifications of the law of war.

After World War II, U.S. military lawyers, trying thousands of defendants before military commissions did, in the words of Justice Robert Jackson, “stay the hand of vengeance and voluntarily submit their captive enemies to the judgment of law” in “one of the most significant tributes that Power has ever paid to Reason.” Reflecting on this distinctive history, one Chairman of the Joint Chiefs of Staff observed that “[t]he laws of war have a peculiarly American cast.” And it is also true that the laws of war have shaped the U.S. Armed Forces as much as they have shaped any other armed force in the world..."
Department of Defense Law of War