US President Donald Trump already has a busy agenda locked in place as he arrives in London on Monday, ahead of two days of ceremonies to mark Nato's founding.
At the US ambassador’s residence in north London and the anniversary venue in Watford, Mr Trump will engage in one-on-one meetings with other leaders. One of the first on the list is French President Emmanuel Macron.
Nato has faced some harsh criticism in recent months, most notably from Mr Macron.
He has been combative before travelling to London, describing the oldest standing multinational alliance as "brain-dead".
The remarks drew concern from the 28 other members of Nato. Recep Tayyip Erdogan, the Turkish President, said it was Mr Macron who should check if he was in a state of brain death.
Mr Erdogan is a disruptive force in the alliance, having bought the S400 air defence system from Russia and tested it using American-made F16 fighters.
A decade earlier, Nato celebrated 60 years with a confident outlook, having grown rapidly after the Cold War and proven its flexibility by extending operations to Afghanistan following the 9-11 attacks on the US.
As the personalities at the top have changed, the world institution is now vulnerable to new political pressures.
Kori Schake, deputy director of the International Institute of Strategic Studies, said the threat was referred to by insiders as the "three presidents problem".
Getting to the end of Wednesday without any of the three taking control of the public narrative would be a major challenge.
Ms Schake warned over Nato’s readiness for a shock, such as a Turkish walkout.
Mr Trump also has form for disrupting Nato summits. His White House did not invent the US campaign for Europeans to pay for a larger share of the defence burden.
Barack Obama made the same argument for states to meet 2 per cent of GDP that was agreed on as a threshold for defence spending.
But Mr Trump consistently and directly challenged other leaders to do more.
Many analysts see the US as far more engaged in the security challenges it faces in Asia. As it pulls back from Europe, splits between the allies become less easy to contain.
Mr Macron’s remarks caused consternation in Berlin, which prioritises the US defence umbrella over a European-only military build-up.
Boris Johnson, the British summit host, has little time to devote to refereeing the disputes that have arrived on his doorstep.
After 100 days as UK Prime Minister, Mr Johnson is in the grips of a make-or-break general election.
Officials hope that the leaders meeting, which will give each country just four minutes to speak, will result in a short declaration.
There is likely to be an important agreement on extending the Nato remit to space and commitments on cyber defence.
For Mr Johnson, at least the end of the meeting on Wednesday and a return to the demands of campaigning for his job could come as a welcome relief.