NATO and collective defence

How Articles 4 and 5 work, whether allies would help and what leaving NATO would mean.

• What is NATO Article 5 – and has it ever been invoked?

  NATO’s Article 5 treats an attack on one ally as an attack on all. What the text says, who decides, why it is not automatic and the one time it was used.

• What would leaving NATO mean for a small country?

  Leaving NATO takes a year’s notice under Article 13. What a small state would lose, what Finland, Switzerland and Israel pay instead, and why no one has left.

• Has NATO’s Article 5 ever been invoked?

  NATO invoked Article 5 once, after 9/11. What the allies did, why Afghanistan was not an Article 5 mission and the crises that stopped at Article 4.

• What does NATO’s Article 4 do?

  Article 4 lets any NATO ally call formal consultations when it feels threatened. What happens next, how it differs from Article 5 and all nine uses since 1949.

• What are NATO’s rules of war and rules of engagement?

  NATO has no rulebook of war of its own. What the 1949 treaty allows, how rules of engagement are set, which laws of war bind allies and what NATO cannot do.