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NATO’s Article 5 Collective Defense Obligations, Explained

Here’s how a conflict in Europe would implicate U.S. defense obligations.

Last Updated: November 15, 2022
Published: March 4, 2022
Geoffroy Van Der Hasselt/Getty
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NATO — the North Atlantic Treaty Organization — is an alliance of 30 European and North American countries, including the United States. Its foundational document is the North Atlantic Treaty, which sets forth NATO’s purpose and obligations: ensuring peace and security through collective defense.

NATO was formed shortly after the end of World War II, at the dawn of the Cold War. The organization’s collective defense obligations, detailed in Article 5, have been invoked only once, on behalf of the United States after 9/11. Russia’s recent invasion of Ukraine has sparked concerns that Russian President Vladimir Putin may expand the scope of the conflict to NATO members like Poland and Lithuania, triggering NATO’s collective defense obligations. Many in the public are now asking what NATO’s collective defense obligations mean for the United States.

What are a NATO member’s collective defense obligations?

Article 5 of the North Atlantic Treaty states:

The Parties agree that an armed attack against one or more of them . . . shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking . . . such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

This language is relatively flexible. It permits each NATO member to decide for itself what action should be taken to address an armed attack on a NATO ally. It does not require any member to respond with military force, although it permits such responses as a matter of international law. A member may decide that instead of responding with force, it will send military equipment to NATO allies or impose sanctions on the aggressor.

If a NATO ally is attacked, would Article 5 authorize the president to send U.S. forces into conflict?

No. Even if a NATO ally is attacked and Article 5 is invoked, the president needs to obtain congressional authorization before sending the military into a conflict zone or otherwise using force. Article 11 of the North Atlantic Treaty explains that “its provisions [shall be] carried out by the Parties in accordance with their respective constitutional processes.” In the United States, that means securing express authorization from Congress, which has the sole constitutional power to declare war and is responsible for military appropriations and oversight.

Consider that treaties are made by the president, with the consent of the Senate. If the invocation of a collective defense treaty automatically allowed the president to use force abroad, the House would be wholly excluded from decisions about where, when, and how the country goes to war. The Senate would play a role secondary to the president. Such a scheme would violate the Constitution’s text and design, which vest “[t]he whole powers of war” in Congress, according to a foundational Supreme Court opinion.

Congress endorsed this analysis in the 1973 War Powers Resolution, a Vietnam War-era law that reaffirms the president’s obligation to seek congressional authorization before using offensive force. The War Powers Resolution states that congressional authorization to use force “shall not be inferred . . . from any treaty heretofore or hereafter ratified.”

What about the president’s inherent powers as commander in chief?

The president’s inherent powers as commander in chief would not allow the president to send the military into a conflict zone or otherwise use military force in response to an invocation of Article 5. The Constitution vests the president with the power to defend U.S. territory and citizens, even without express authorization. But it does not permit the president to use force against an adversary who poses no direct threat to the United States, as would be involved in a military campaign to assist a NATO ally.

Since the Cold War, executive branch lawyers have tried to broaden the scope of the president’s inherent powers. They have argued that the Constitution permits the president to defend not only U.S. territory and citizens but also more abstract national interests, such as the credibility and effectiveness of the United Nations. As many experts have noted, this open-ended “national interest” theory is constitutionally dubious.

Still, executive branch lawyers concede that the president cannot unilaterally commit the military to a conflict of substantial nature, scope, and duration, even if there is a strong national interest. Any military confrontation between Russia and NATO would surely be of a substantial nature, scope, and duration — and would therefore require congressional authorization. This limitation on the president’s inherent powers explains why President George W. Bush sought congressional authorization for the Afghanistan War and the Iraq War, large-scale conflicts involving ground forces.  

What could Congress’s response to an invocation of Article 5 look like?

If Congress were to decide that a military response is “necessary,” Congress could declare war or, more likely, adopt a limited authorization to use force. For years, experts and advocates have agreed that any authorization to use force should specify the conflict’s purpose and geographical scope, as well as the identity of the enemy, and that it should include an expiration date. These limitations ensure that Congress reviews the authorization on a regular basis and understands where, why, and against whom U.S. forces are fighting.

Would waiting for Congress conflict with our obligations to aid our NATO allies?

No. Our NATO allies understand that legislatures play an important role in determining what kind of support is “necessary” to respond to an invocation of Article 5. After 9/11, NATO’s governing body invoked Article 5 and called upon the NATO allies to support the United States in its response to the terrorist attacks. In turn, the leaders of NATO allies like Germany asked their legislatures for permission to deploy forces. On November 16, 2001, the German Bundestag voted to commit 3,900 troops to fight in Afghanistan as a means of fulfilling its Article 5 obligations.

Moreover, Congress can act quickly in response to national security developments, and it would likely do so for any invocation of Article 5. Congress passed the 2001 Authorization for Use of Military Force, the congressional authorization to pursue those responsible for 9/11, on September 14, 2001. In 1964, it passed the Gulf of Tonkin Resolution just three days after the supposed incident that prompted President Lyndon Johnson’s request for authorization to use force in Vietnam.

What would happen if the president sent the military abroad without securing congressional authorization?

If the president were to send the military into a conflict zone without congressional authorization, Congress could invoke the War Powers Resolution. The War Powers Resolution provides that military forces operating “without a declaration of war or specific statutory authorization . . . shall be removed by the President if the Congress so directs.” Congress could also use its power over military appropriations to restrict the president’s use of funds on an unlawful war.

What does the invocation of Article 4 mean?

In February 2022, NATO members invoked Article 4 of the North Atlantic Treaty in response to Russia’s invasion of Ukraine. Article 4 permits members to call a NATO meeting when they perceive a threat to the “territorial integrity, political independence or security” of any NATO ally. The invocation of Article 4 does not trigger any collective defense obligations.

Consistent with Article 4, the leaders of each NATO member, including President Biden, convened to reaffirm their commitment to Article 5. As a result of the meeting, NATO members made “additional defensive deployments” to the easternmost allies, some of which share a border with Ukraine. NATO members did not deploy or commit to deploying forces to Ukraine.