This assistance is taken forward in concert with other Allies. It is not necessarily military and depends on the material resources of each country. It is therefore left to the judgment of each individual member country to determine how it will contribute. Each country will consult with the other members, bearing in mind that the ultimate aim is to “to restore and maintain the security of the North Atlantic area”.

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.

The principle of providing assistance

With the invocation of Article 5, Allies can provide any form of assistance they deem necessary to respond to a situation. This is an individual obligation on each Ally and each Ally is responsible for determining what it deems necessary in the particular circumstances.

This assistance is taken forward in concert with other Allies. It is not necessarily military and depends on the material resources of each country. It is therefore left to the judgment of each individual member country to determine how it will contribute. Each country will consult with the other members, bearing in mind that the ultimate aim is to “to restore and maintain the security of the North Atlantic area”.

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At the drafting of Article 5 in the late 1940s, there was consensus on the principle of mutual assistance, but fundamental disagreement on the modalities of implementing this commitment. The European participants wanted to ensure that the United States would automatically come to their assistance should one of the signatories come under attack; the United States did not want to make such a pledge and obtained that this be reflected in the wording of Article 5.

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