Here are five things to know about the US-led defence club, which was set up during the Cold War to protect western Europe against Soviet aggression.

- All for one -

The core of the NATO treaty is Article 5 which states that allies agree "that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all."

If one of the allies were to invoke the article, and the other allies are unanimous in agreeing that the member is indeed under attack, each will take "such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area."

During the Cold War, this principle translated as an effective US security guarantee for smaller allies facing the implied threat of the Soviet Union and its Warsaw Pact allies in Europe.

But it has never been invoked for that purpose.

- One for all -

In fact, Article 5 has only been invoked once, to defend the United States.

In October 2001, just weeks after Al-Qaeda members hijacked four airliners and crashed them into targets in New York and Washington DC, the alliance rallied to America's aid.

While the US military response was dominated by its own troops under its own command, NATO AWACS reconnaissance planes were deployed to US skies and warships headed to the eastern Mediterranean.

- Growth spurt -

At its birth in 1949, NATO was an alliance of North American and western European democracies, facing their Communist foes across the Iron Curtain.

But after the Berlin Wall fell many former Moscow satellites came knocking on NATO's door, infuriating Russian President Vladimir Putin.

NATO's members already include Latvia, Estonia and Lithuania, which directly border Russia. Ukraine and Georgia are seeking to join too.

Sweden, which applied for membership at the same time as Finland, has jumped ahead of Ukraine in the queue, but its application has run into opposition from Turkey, which accuses Stockholm of sheltering suspected Kurdish militants it wants to prosecute.

- French leave -

France has had a complicated relationship with the alliance, despite being one of its founding members.

World War II hero president Charles de Gaulle was distrustful of NATO's US leadership and pulled France out of the alliance's military command structure in 1966.

It was 43 years before president Nicolas Sarkozy took France back to full membership, in return for the promise of prestigious commands for French officers.

But in 2019, France again struck a discordant note, with President Emmanuel Macron declaring the alliance to be in the throes of "brain death".

- Funding problems -

The alliance has been dominated by the United States from the outset, in part because the superpower's defence budget dwarfs that of all the other members combined.

In recent years Washington has accused its European allies of not pulling their weight -- former president Donald Trump was particularly critical -- and pressured them to increase their contributions.

In 2014, members agreed to aim to increase their individual defence budgets up to two percent of their national GDP within a decade.

Spending has increased, but in 2022 only seven members met the target, according to NATO: Britain, Estonia, Greece, Latvia, Lithuania, Poland and the United States.

Germany last year announced plans to massively increase defence spending, but it only expects to meet the two-percent target in 2025.