Hailed by its leadership as "the most successful alliance in history", it now includes superpower the United States, Canada and 27 European partners, old and relatively new.

Here are five things to know as Donald Trump, Boris Johnson, Angela Merkel, Recep Tayyip Erdogan and their colleagues prepare for the big event.

- French leave -

France has had a complicated with its allies, even before current president Emmanuel Macron declared NATO "brain dead" in an interview earlier this month.

France was a founder member of the alliance with the North Atlantic Treaty was signed in April 1949.

But president and former general Charles de Gaulle, distrustful of US leadership, 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, winning by doing so a promise of prestigious commands for French officers.

- 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 came to America's defence.

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 pact -

At its birth, NATO was an alliance of North American and western European democracies, facing their Cold war communist foes across the Iron Curtain.

But after the Berlin wall fell many former Moscow satellites in eastern Europe turned westwards, to the fury of post-Soviet Russia.

NATO's 29 members now include for example the three Baltic republics -- Latvia, Estonia and Lithuania -- which have tense borders directly with Russia.

The next new member is to be North Macedonia, a former Yugoslav republic which should be ratified as a member as soon as Spain elects a government that can add its signature to the accession.

- Money 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.

But Washington has assumed security responsibilities far beyond the North Atlantic theatre, and President Trump has repeatedly accused European allies of not pulling their weight.

In 2014, under Trump's predecessor Barack Obama, members agreed to aim to increase their individual defence budgets up to two percent of their national GDP -- but only nine have done so.

Nevertheless, NATO secretary general Jens Stoltenberg confidently forecasts that Canada and European members will have spent between them 400 billion dollars between 2016 and 2024, bringing them close.