The laws of war fall into two basic types: one set governs whether it is legal for a state to go to war against another, or jus ad bellum. The other set lays out how all actors should conduct themselves in the midst of fighting, or jus in bello.

The Right to Go to War

Jus ad bellum derives from “just war” doctrine dating at least back to the work of St. Thomas Aquinas in the 1200s. The concept of just war re-emerged after Europeans fought each other for multiple decades, culminating in the treaty of Westphalia in 1648. It emerged again after hundreds of millions of people perished in World Wars I and II. Today, using military force against a sovereign state is considered “just” for only three reasons under international law:

1. In self-defense;
2. When one state asks another state to send troops (for example, Russia legally sent troops to Syria by invitation of the Assad government); and
3. If the U.N. Security Council determines that the war is legal under Article 51 of the U.N. Charter (for example, after Saddam Hussein invaded Kuwait, the Security Council authorized a multinational military response).

Russia’s war against Ukraine does not meet any of the criterion for just war. NATO and EU institutional expansions, while possibly undiplomatic, did not constitute acts of war against Russia. In contrast, Ukraine is confronting kinetic military aggression waged by a neighboring state. Ukraine’s resort to self-defense, and invitations for external military assistance are legal under international law. The U.N. Security Council has been stymied because Russia is a permanent, veto-wielding member.

Historically, weakening sovereignty norms have sparked the deadliest periods in world history. The fundamental danger of Putin’s war of aggression against Ukraine is that it opens the door for other states to violate the principles of sovereignty and jus ad bellum. Without these principles, inter-state war threatens every state, and thus every individual, in the world.

Right Conduct During War

Jus in bello, also referred to as international humanitarian law (IHL), applies to all warring parties irrespective of the reasons for the conflict. It does not speak to whether or not the cause of war is just. Instead, this body of law seeks to protect war victims and their fundamental rights, no matter to which party they belong. The four Geneva Conventions constitute the central pillars of IHL:

1. The first convention, dating back to 1864, stipulates that the sick and wounded should be protected impartially, and that medical facilities should not be targeted during fighting.
2. The second extends the first convention to the shipwrecked.
3. The third requires that all warring parties treat prisoners of war humanely, and that prison camps should be open to inspection by neutral countries or entities.
4. The fourth convention, agreed on the heels of World War II in 1949, requires U.N. member states to punish those who commit crimes such as unlawful killing, torture, serious bodily injury or suffering, unlawful deportation (ethnic cleansing), unlawful confinement, and gender-based crimes such as rape and forced prostitution. The fourth convention includes three additional protocols, extending civilian protections to victims of wars against racist regimes, wars of self-determination and internal conflicts.

Genocide

A separate “Convention on the Prevention and Punishment of the Crime of Genocide” known simply as the genocide convention, lists five acts that can be punishable as genocide. These can be committed in the context of war, or not. Genocide is based on one party’s “intent to destroy, in whole or in part, a national, ethnical, racial or religious group.” Intent is the main bar to determine whether genocide is taking place, and not the overall number of people killed. The acts include causing serious bodily or mental harm to members of the group; deliberately inflicting conditions of life calculated to bring about the group’s physical destruction in whole or in part; imposing measures intended to prevent births within the group; and forcibly transferring children of the group to another group. Article 1 notes that the contracting parties to the convention undertake not just to punish, but to prevent, genocide. 

Russia’s Crimes

In surveying these various crimes, it would seem that Russia today is committing three essential types of crimes: a war of aggression, war crimes and genocide. Early in the war, a majority of member states in the U.N. General Assembly recognized Russia as the aggressor. Additionally, even though U.N. Security Council action is hampered because of Russia’s veto power, at a Security Council meeting on September 27 several states charged Russia with jus ad bellum violations. In terms of Russia’s crimes while conducting the war, the International Criminal Court and the U.N.’s Independent International Commission of Inquiry on Ukraine have documented evidence of acts that appear to constitute war crimes, such as indiscriminate killings and sexual and gender-based violence against civilians. Finally, more than one dozen states have submitted documentation to the International Court of Justice with allegations of genocide being committed by Russia in Ukraine.

Although all of these are horrendous crimes, the crime of aggression in particular threatens the very fabric of the international laws that enable all states to exist without constant threats to their borders. Upholding international law makes peace possible. Russia’s leaders must be held to account.

Dr. Lise Morjé Howard is a senior scholar in residence at the U.S. Institute of Peace. She is also a professor of Government and Foreign Service at Georgetown University (on leave, 2022-23) and the president of the Academic Council on the United Nations System.