Yes, once: NATO invoked Article 5 after the attacks of 11 September 2001 on the United States. No other attack has been declared an Article 5 case.
The clause written to bring America to Europe’s defense was used, the only time, by the other allies for America.
12 September 2001
The day after the attacks on New York and Washington, the North Atlantic Council met in Brussels. The allies agreed that “if it is determined that this attack was directed from abroad against the United States, it shall be regarded as an action covered by Article 5.” The condition mattered: Article 5 covers an armed attack on an ally, and the alliance first wanted to know where this one had come from.
On 2 October, after a briefing on the results of the investigation, the allies confirmed that the attacks were covered by Article 5. Two days later they agreed a package of eight measures: sharing intelligence, helping allies facing a greater terrorist threat, protecting US and allied facilities, replacing allied assets withdrawn for the campaign, blanket overflight rights, access to ports and airfields, and readiness to deploy NATO’s naval and airborne early-warning forces.
What the alliance actually did
From 9 October 2001 seven NATO AWACS radar planes flew over the United States in Operation Eagle Assist. Their 830 crew members came from 13 countries; by the time the operation ended on 16 May 2002 they had flown over 360 sorties and nearly 4,300 hours.
At sea, NATO ships began patrolling the eastern Mediterranean on 6 October 2001, in what became Operation Active Endeavour. It was launched as an Article 5 operation and ran until October 2016.
The war in Afghanistan was not an Article 5 operation. The NATO-led ISAF mission, which the alliance commanded from August 2003, was a UN-mandated force deployed at the request of the Afghan authorities.
The cases that did not become Article 5
Several crises since then came close to the question without reaching it.
- Article 4 instead. Allies have asked for consultations under Article 4 nine times, among them Türkiye after Syria shot down one of its jets in 2012, eight eastern allies on the day Russia invaded Ukraine in 2022, and Poland and Estonia after Russian drones and fighter jets crossed their borders in September 2025.
- Ukraine. Russia’s invasion in 2022 did not trigger Article 5 because Ukraine is not a member, and Article 5 protects members only.
- Paris 2015. After the terrorist attacks of 13 November 2015, France turned to the EU’s mutual assistance clause, Article 42(7) of the Treaty on European Union, and not to NATO. It was the first time any EU state had activated that clause.
- Hybrid attacks. NATO says cyber and hybrid attacks could lead to Article 5, but none of the sabotage, cable cuts or drone incursions of recent years has been treated that way. The record is in What is hybrid warfare?
Why it has been used only once
Invocation needs the agreement of all allies in the North Atlantic Council, and no armed attack on an ally since 2001 has put that question to them. Below that line allies use consultations, reinforcements and national measures. The legal basics are in What is NATO Article 5?
No European ally has yet had to call on it. Keeping it that way depends on allies who visibly can, and will, act together; what they spend to make that credible is in European defence in numbers.