On February 24, 2022, Russian tanks rolled across international borders into Ukraine with the aim of conquering an independent sovereign state and member of the United Nations.

Not only was this a breach of the prohibition on the use of force — a core principle of international law and the United Nations Charter stating that members cannot threaten or use force against other states — it was also the most serious breach since the prohibition was codified in 1945, according to Mary Ellen O’Connell, the University of Notre Dame’s Robert and Marion Short Professor of Law and professor of international dispute resolution in Notre Dame’s Kroc Institute for International Peace Studies.

In O’Connell’s new article, “What Remains of Law Against War,” published in the Georgetown Law Journal, she states, “The law against war appears to be dead.”

O’Connell — author of “The Crisis in Ukraine” and editor of “What is War?” — also served as a professional military educator for the U.S. Department of Defense and chair of the International Law Association’s Committee on the Use of Force. She places the Russian invasion of Ukraine at the top of a long list of international lawbreakers worldwide.

“Violations in the Middle East, Africa, and elsewhere are plain,” she writes.

According to O’Connell, these include the United States-led invasion of Iraq in 2003, which had no legal justification; repeated attacks on Syria by Turkey, Israel and the U.S. based on a distorted view of self-defense in the United Nations Charter; and Israel’s use of major force against Palestine in defiance of a 2004 decision of the International Court of Justice (ICJ). Uganda’s incursions into the long-running civil war in the Democratic Republic of Congo were found by the ICJ to violate the Charter in 2010. Rwanda has been committing the same violation in 2025.

O’Connell also notes North Korea’s repeated missile strikes on South Korea and China’s aggressive behavior toward other states in pushing its unfounded claims to maritime space. The U.S. fought in Afghanistan for 20 years on the excuse that Afghanistan was responsible for 9/11. It was a false claim in fact and, therefore, in law, O’Connell says. The U.S. has carried out repeated missile strikes on Pakistan without justification. India also has attacked Pakistan in plain defiance of the United Nations Charter.

O’Connell says, “The serious and widespread breaches have resulted in large part because of the theory of legal positivism. Positivism holds that there is no lasting and immutable natural law, only the law ‘posited’ by lawmakers. When it comes to international law, positivists argue that states with major militaries can change the law through their conduct. The result is that no law is safe from the subject choices of the few. In fact, the prohibition on force is no mere positive law rule. It is a peremptory norm of natural law, which does not shrink or disappear.”

Analyzing how the law against war became so disrespected, O’Connell points to specific causes of its decline dating back to the end of the Cold War when, she says, the United States and democratic allies began asserting privileges to use military force that are clearly forbidden under the United Nations Charter.

“Two Princeton University professors attempted to justify the violations by creating a system in competition to the charter and international law in general,” O’Connell says. “In their system, the U.S. as a state with unequaled military power and democratic values has the imperial right to determine for itself and its allies when resort to force is lawful. The Princeton alternative became known as the Western International Rule-Based Order (RBIO). President Biden particularly relied on it in condemning Russia’s invasion of Ukraine, only to be called out for pursuing a double standard and exhibiting hypocrisy.”

Her paper explains that RBIO legal justifications are based on manipulating positive law and ignore the fact that peremptory norms are natural law (humans are born with a moral compass that guides behaviors). She suggests applying social science research on “norm death” to help reverse years of disrespect for the law against war.

“Russia’s attempt to conquer Ukraine demonstrates how weak the taboo against resort to war has become,” she says. “Social scientists show that building respect for normative ideas like law can occur when leading states model compliance with the law.”

International law experts today define a “leading state” as one that complies with the law, but O’Connell points out that the law such states are modeling must be real international law. This is law that has its origins in the ancient concept of natural law.

“This is the opposite of law made up to attempt to place a legal fig leaf over an invasion or missile strike or to promote U.S. military hegemony,” O’Connell says. “President Trump is defying even the RBIO system of legal privilege. He is presenting the world with a blank slate where we can rewrite the wisdom of the ages or allow lawless, violent chaos to prevail.”

O’Connell’s “Speaking Truth to Trump on the International Rule of Law” is available on the blog of the European Journal of International Law.

Contact: Mary Ellen O’Connell, 574-631-4197, maryellenoconnell@nd.edu