Article 5: How NATO's collective defence clause works - and the one time it has been invoked

President Donald Trump's visit to the NATO summit on Wednesday is being closely watched for one big reason: will he commit America to Article 5?

The principle of collective defence - enshrined in Article 5 of the alliance's founding treaty - goes to the heart of what NATO is set up to do.

Under Article 5, an attack on one member is considered an attack on all. But it's not quite that simple.

What is Article 5?

Article 5 is the core principle of the 32-member North Atlantic Treaty Organisation.

It says that an armed attack against one or more of the members shall be considered an attack against all members.

It further states that if such an armed attack happens, each other member would take "such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area".

Article 5 covers the territory of member states in Europe and North America as well as Turkey.

It doesn't cover bases in foreign countries or territories outside the alliance area - for instance, it didn't apply to the Vietnam War, the Falklands or to the recent Iranian attack on a US base in Qatar.

How does Article 5 work?

This is where it gets a bit more complicated.

When Article 5 is invoked by a NATO member, allies can provide any form of assistance they deem necessary to respond to a situation.

It doesn't automatically mean military assistance and it's up to each individual ally to decide what action they think is necessary.

That assistance is then taken forward in conjunction with the rest of the alliance.

This was a tension between Europe and the US at the founding of NATO - the US did not want to sign itself up to automatically provide assistance - and it's very similar to what we are seeing with President Trump now.

Read more: Trump dodges commitment to founding NATO principle

Has Article 5 ever been invoked?

Yes, once. After the September 11 attacks, NATO allies triggered Article 5 and came to America's assistance.

This culminated in the NATO-led invasion of Afghanistan due to accusations that the ruling Taliban had provided safe haven to al Qaeda.

NATO allies fought side-by-side with the US and in some of the fiercest parts of the campaign.

They shared the burden of those who did not come home too.

It's why JD Vance's remarks about "some random country that hasn't fought a war in 30 or 40 years" were viewed as particularly insulting in Europe.

NATO allies have also acted to bolster collective defence measures on other occasions, beneath the threshold of Article 5, for example following Russia's annexation of Crimea and the 2022 invasion of Ukraine.

(c) Sky News 2025: Article 5: How NATO's collective defence clause works - and the one time it has been invoked