It’s a rare moment offering up a world of interpretation for body language experts as the globe’s most powerful statesmen and women jostle for position on a [Nato](http://What is Nato, what will be discussed and how much does it cost?) stage.
And at the 70th edition of the North Atlantic Treaty Organisation, hosted on the outskirts of Watford, there are likely to be some sparks flying within a 29-nation military alliance which has been more than a little at odds with each other in recent years.
So with the leaders posing together for a pre-meeting photograph, who was being watched particularly closely for telling evidence of uncordial relations?
Donald Trump and Emmanuel Macron
The French president criticised recent Nato get-togethers for being dominated by talk of ways to lessen the financial cost on the US and said the organisation is “brain dead”. Donald Trump said that was “very insulting” but Mr Macron has stood by the description.
The Franco-American war of words also fizzled after Mr Macron criticised Turkey's recent military action against the Kurds in Syria - taking advantage of a US troops withdrawal - without warning other Nato members.
Mr Trump said that was a "very, very nasty statement essentially to 28 countries" and told Mr Macron to worry about events closer to home. "I think they have a very high unemployment rate in France. France is not doing well economically at all,” he pointedly added.
Emmanuel Macron and Recep Tayyip Erdogan
See above. Mr Macron went further than others in condemning Turkey’s “crazy” attack in October on the Kurds, who are seen as a key Western ally in the fight against so-called Islamic State.
The Turkish president hit back, accusing Mr Macron of “a sick and shallow understanding” of terrorism, suggesting he was the one who was “brain dead”.
Boris Johnson and German Chancellor Angela Merkel have also sought to raise the issue of Turkey’s incursion, but in more moderate language. Turkey also angered many Nato members after buying a Russian air defence system so Erdogan is unlikely to be welcomed with open arms.
Boris Johnson and Donald Trump
No war of words here but the prime minister will be mindful not to glad-hand the US president too much amid an election where his rivals are all framing their future trading relationship as damaging for the UK post-Brexit.
Mr Trump has already openly endorsed Mr Johnson’s credentials for office - although on Tuesday confirmed he can work with any future prime minister - so the Tory campaign teams are desperate for their man not to gift their opponents a chummy photo opportunity.
Mr Johnson attempted to draw a line last week when he said “close friends and allies” like the UK and the US should not get involved in each other’s elections.
Donald Trump and 23 leaders
In his State of the Union address earlier this year, Donald Trump said the US had been “treated very unfairly by friends of ours, members of Nato” over a period of years.
He’s U-turned on claims the alliance is “obsolete” but wants the other members to pay more towards Nato defence spending (with the US spending more than twice as much on defence each year as the rest of Nato combined).
Nato has agreed a new formula to share budget costs though many Nato governments are still below the agreed commitment to spend at least 2% of GDP on defence.
Aside from the UK and the US, the only other countries meeting the 2% target are Greece, Poland, Latvia and Estonia. So there are 23 leaders who were probably not desperate to be standing next to the outspoken Commander In Chief.