Boris Johnson will attempt to bring harmony to the top of NATO by reminding allies that it is “one for all, and all for one” in the quest to keep their people safe.
The prime minister’s words – echoing the famous phrase from French author Alexandre Dumas’s The Three Musketeers – will particularly resonate with Emmanuel Macron.
Following talks in Downing Street on Tuesday, the French president insisted he stood by his previous description of NATO as suffering from strategic “brain death”.
US President Donald Trump branded Mr Macron’s comments as “very nasty” before the pair met at the US ambassador’s residence in London.
Mr Johnson will address leaders from the 29 NATO member states later at The Grove in Hertfordshire.
He is expected to say: “Seventy years on, we are rock solid in our commitment to NATO and to the giant shield of solidarity that now protects 29 countries and nearly a billion people.
“The fact that we live in peace today demonstrates the power of the simple proposition at the heart of this alliance: that for as long as we stand together, no-one could hope to defeat us – and therefore no-one will start a war.
“This essential principle is enshrined in Article 5 of the North Atlantic Treaty – that if any one of us is attacked, all of us will go to their defence.
“If NATO has a motto, it is, ‘one for all, and all for one’.”
The meeting is expected to consider new threats, including in the areas of cyber and space, after the alliance last month declared space one of its operational domains alongside air, land, sea and cyber.
Earlier on Tuesday the US president vowed to stay out of the general election – before quickly confirming “I’m a fan of Brexit” and saying Mr Johnson was “very capable” and would “do a good job”.
Later the leaders gathered at Buckingham Palace and Downing Street to mark 70 years since NATO was formed.
Their smiles for the traditional “family photo” came despite divisions between some of the premiers and protests attended by nurses and doctors concerned by potential risks to the NHS from a future US-UK trade deal.
The government says the health service will not be on the table when it comes to any trade deal struck with the US.
Mr Johnson was notably absent from the steps of 10 Downing Street to welcome his guests, possibly wary of the impact of being pictured with Mr Trump ahead of the general election next Thursday.
Analysis by Sam Coates, deputy political editor
It’s been quite an interesting spectacle – why? Because we’ve had some comings and going from day one of this two-day summit. But no sign of Boris Johnson.
He’s made world leaders walk the walk up the red carpet to the Number 10 door but he’s not at any point been there to greet them.
It was quite interesting watching Donald Trump having to make his own way in with no one to bounce off in front of that black door.
He just was a world leader like any other – he turned around and went inside.
These events – NATO summits – are big marquee moments for the hosts – so it is extraordinary that we’ve not had any sign of the British PM in public in any of them.
Of course it’s a reflection of the fact we’ve got an election in nine days’ time.
World leaders have gathered at Buckingham Palace and Downing Street to mark 70 years since the North Atlantic Treaty Organisation (NATO) was formed. US president Donald Trump, French leader Emmanuel Macron and German Chancellor Angela Merkel were among those to mingle with the Queen and Prime Minister Boris Johnson as the two-day summit began in London.