In the wake of World War II, as European states and the US bargained over their future, they came to a compromise — the Old World got Article 5 with its collective defense pledge, and the US got Article 3, which pledges “continuous and effective self-help and mutual aid.”  

Article 5, the best-known part of the treaty, was very carefully worded by the US to ensure no aggressive European action could drag it into a war. It was also designed to prevent the US from having to defend the European colonies, which at that point covered much of the globe. 

Article 5 reads, in full:  

“The Parties agree that an armed attack against one or more of them in Europe or North America 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-defense recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.” 

There are three major elements to the article. First, there needs to be an armed attack. Second, “they agree that” means all NATO allies vote on whether it is in fact an armed attack. Third, “such action as it deems necessary” means each ally gets to decide how they will respond.  

Under international law, an “armed attack” usually requires one of the following: property is damaged, property is destroyed, people are hurt, or people are killed. If an operation does not achieve one of those effects, it is difficult to convince others that an armed attack has occurred. Without that as a pre-requisite, there is no case for invoking mutual defense.  

The “they” who vote comprise the North Atlantic Council (NAC), which includes all the allies. Each has an ambassador in Brussels to vote for them on routine measures, but heads of state and government can also attend in person, as is usual during summits.  

All must agree that an armed attack has occurred and decide whether it meets the threshold for a NATO response. Any of the 32 member states can veto a motion to invoke Article 5 and, because it is a vote by politicians, there are many considerations to their decision-making that cannot be codified into a checklist.  

“Such action as it deems necessary” means each ally gets to make an individual decision on their response. For example, an ally could vote in support of invoking Article 5 and then proceed to provide only rhetorical support.  

While Article 5 has only been invoked once, in support of the US after the 9/11 terror attacks, it has been requested on seven other occasions, and in each case, the allies “coordinated collective defense measures.”  

In several of these cases, the NAC decided to invoke Article 4 (consultation) instead. Three, all involving Turkey, then led to concrete action: Patriot missiles were deployed to the country during the 1991 Gulf War; it was supported with a package of defensive measures and Operation Display Deterrence in 2003; and in 2012 Patriot missiles were again stationed there in response to escalation in Syria.  

Hypothetical scenarios illustrate the issues involved. Imagine these situations involving Russia and Finland. 

Armed Russian aircraft cross into Finnish airspace, armed Russian patrol boats enter Finnish waters, or Russian soldiers cross the Finnish border — none would make the case for invoking Article 5.  

Russian ground forces shooting at Finnish forces might be sufficient for NATO to invoke the article, (although the NAC responded with an Article 4 process when Turkish troops came under fire) but it would certainly see someone — probably, but not necessarily Finland — citing Article 5 and requiring an NAC vote.  

There is no inevitability in the Article 5 process. Each ally votes on whether or not to invoke it, casting a veto if they choose, and then each decides their response. Taken together, this prevents states from being drawn into a war against their will.  

As a corollary, a “sub-Article 5” scenario allows coalitions of the willing, something rarely discussed in NATO terms but perfectly plausible. In terms of the Finnish hypothetical above, for example, the UK, France, the Netherlands, and Sweden might send troops, with US logistic and intelligence support. The response can be tailored to fit the circumstances. 

G. Alexander (Alex) Crowther, PhD, is a Non-resident Senior Fellow with the Transatlantic Defense and Security Program at the Center for European Policy Analysis (CEPA). He has published in a variety of formats and locations since 2005, mainly on cyber and European security issues.  

Europe’s Edge is CEPA’s online journal covering critical topics on the foreign policy docket across Europe and North America. All opinions are those of the author and do not necessarily represent the position or views of the institutions they represent or the Center for European Policy Analysis.

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