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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: March 8, 2022
Published: March 4, 2022
Geoffroy Van Der Hasselt/Getty

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 recent invoc­a­tion of Article 4 mean?

Several NATO members recently 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 on Febru­ary 25 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.