Does a President’s War of Choice Obligate Treaty Partners to Join the Cause? Light from History

When a nation attacks another country and wages offensive warfare—a war of choice—are treaty partners in a defense alliance obligated to support the aggressor nation in the conduct of the war?  If you listen to President Donald Trump, and members of his administration, the answer is clearly “yes.” Thus it is that Trump has expressed toward members of NATO increasing frustration and anger, to the point that he declared on March 31, that if they do not join the United States in opening the Strait of Hormuz, that they would have to fend for themselves, and get “your own oil and fight your own battles.” Trump has gone further in his threats, telling the British newspaper, The Telegraph, that he may withdraw the United States from NATO. Indeed, he said, “it is beyond reconsideration.”

President Trump’s threat to withdraw from NATO may well be a bullying ploy to coerce European allies into doing his bidding, but in any case, such a move would violate American law. In December 2023, Congress passed legislation that was included in the 2024 National Defense Authorization Act that bars any president from unilaterally withdrawing from NATO without the approval of two-thirds of the Senate or an Act of Congress.  The measure was designed, specifically, to prevent Trump from withdrawing from NATO, which he threatened to do on several occasions in 2019. The measure also grants Congress legal standing to challenge in court any effort by Trump to withdraw from NATO, which creates a high legal hurdle for the executive.

The legal question surrounding Trump’s assumption that NATO members, per Article V of the treaty, have an obligation to support his war of choice in Iran, is easily settled.  Since Trump initiated or commenced the war in Iran, NATO members have no legal obligation to join the war. Article V of the North Atlantic Treaty, the centerpiece of the “collective defense” treaty, provides that an armed attack against one member in Europe or North America is considered an attack on all.  The only time that that Article V has been invoked was after the 9/11 attack when NATO members rushed to the aide of the United States. Article V would have been triggered, if Iran had commenced war against the U.S.

The question of the obligation of a defense alliance partner to support an aggressor’s war of choice was raised, and settled, in 1793, when Alexander Hamilton, Secretary of the Treasury, defended President George Washington’s Proclamation of Neutrality in the war between France and England. The US had signed in 1778 the Treaty of Alliance with France, essentially a mutual defense treaty, much like NATO. When France initiated war, the question arose: Is the U.S. required to defend France? Hamilton, invoking what he called unanimity among authorities on the Law of Nations, declared, emphatically, “no.” He wrote that resolution of issue depends on which nation “first declares or actually begins a War.” He added: “Tis the commencement of the War itself that decides the question of being on the offensive or defensive.” Since France had commenced the war, “the United States are free to refuse a performance of the guarantee, if asked.”

By what reasoning may one sovereign nation initiate war and demand other sovereign nations to join the war effort? Hamilton cited Vattel, whom America’s founders regarded as the authority on all questions of international law. Vattel “justly observed,” wrote Hamilton, “that it does not belong to any foreign Power to take cognizance of the administration of the sovereign of another country, to set himself up as a judge of his conduct or to oblige him to alter it.” The claim by one nation of authority to dictate or drag others into its “offensive war,” Hamilton, borrowing from Vattel, “would disturb the tranquility of nations, to excite fermentation and revolt everywhere.” France, in 1793, like Trump in 2026, lacked such authority.

No treaty at international law obligates a signatory state to defend a nation that has commenced war. In fact, no treaty in U.S. history has purported to create such an obligation. There is no principled reasoning in the assumption that sovereign states can be held hostage by another’s decision to start a war. Such an admission would amount to surrender of a nation’s sovereignty.