London: US President Donald Trump and his Nato counterparts gathered in London Tuesday to mark the alliance’s 70th birthday amid deep tensions as spats between leaders expose a lack of unity that risks undermining the military organisation’s credibility.

For the third summit in a row, Trump is expected to renew demands that European allies and Canada step up defence spending. Meanwhile, lamenting Nato’s “brain death” due to a lack of US leadership, French President Emmanuel Macron said Nato needs “a wake-up call”.

What are all the arguments all about?

Macron insists that strategic questions must be addressed, like improving ties with Russia and how to handle an unpredictable ally like Turkey.

In turn, Turkish President Recep Tayyip Erdogan has lashed out at Macron, and their very public argument bodes ill for a summit hosted by British Prime Minister Boris Johnson, who is deep into an electoral campaign and desperately wants to smooth things over.

Ankara raised the ire of its allies by invading northern Syria, and for buying Russian air defence systems with powerful computers aboard that suck up data and would compromise the military equipment of allies if they were stationed nearby.

What’s the Turkish connection?

Before heading to London, Erdogan suggested that Turkey might not back Poland and Nato’s Baltic allies — Estonia, Latvia and Lithuania — should they require defending unless the allies support Turkish concerns about Syrian Kurdish fighters, which Ankara sees as terrorists.

What is the Collective Defence clause?

That threat raises new questions about Nato’s commitment to its collective defence clause — Article 5 — under which all allies vow to come to the aid of a member under attack. The clause has only ever been activated once, after the 9/11 attacks in the United States. The two-day summit kicked off with receptions at Buckingham Palace and Downing Street. A short working session will be held at a golf resort in outer London on Wednesday.

What does the Nato chief have to say about the spat?

Nato Secretary-General Jens Stoltenberg said on Tuesday it was not right to question the Western alliance’s security guarantee but that he was working to solve the dispute with Turkey.

Asked if the issue could be resolved by the end of the London summit, he said: “I will not promise that, but what I can say is that we are working on that. But it is not like Nato doesn’t have a plan to defend the Baltic countries.” Speaking after a breakfast with Trump, Stoltenberg said Nato still had strong support in the United States and cited a “big paradox” that while people were questioning the transatlantic bond, citizens backed Nato. “We have bad rhetoric but extremely good substance,” he said.

Will the unity hold after the summit?

Even Stoltenberg conceded that “we should never question the unity and the political willingness to stand together and to defend each other. The whole purpose of Nato is to preserve peace. It’s to prevent conflict by sending a clear message to any potential adversary that if one ally is attacked it will trigger a response from the whole alliance”. The spats between leaders threaten to expose a lack of unity that could undermine the military organisation’s credibility.