Deterrence value
Supporters say the Article 5 guarantee discourages potential adversaries from attacking any member, preventing wars before they start.
In plain English
When an attack on one means all.
It's the NATO rule that says if one member country is attacked, the others have to help defend it.
Simple example
Article 5 has been invoked only once in NATO's history — by the United States after the September 11, 2001, terrorist attacks, leading allies to contribute forces to operations in Afghanistan.
Why it matters
What the term actually changes.
Risk of entanglement
Critics warn the clause could draw the United States into conflicts started by smaller allies, with the scope of the obligation left partly to each country's discretion.
How it works
The mechanics, in practice.
Triggered by attack
Any member that suffers an armed attack can request the alliance invoke Article 5, prompting consultations among all members.
Flexible response
Each country decides what assistance to provide — which can range from military force to logistics or intelligence — rather than being required to send troops automatically.
Related guide
Should the United States remain in NATO?
A look at the 75-year-old alliance, what membership costs and commits the U.S. to, and the case on each side.
Read the guide → Related issue brief
Should the United States remain in NATO?
Lawmakers and voters are debating whether continued U.S. membership in the 32-nation alliance still serves American interests.
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Should the United States remain in NATO?
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