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.
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.
Mr Johnson was notably absent from the steps of 10 Downing Street to welcome guests, possibly wary of the optics of being pictured with Mr Trump ahead of the general election next Thursday.
Labour leader Jeremy Corbyn had also been invited to an earlier reception at Buckingham Palace, where it was suggested he could raise his fears about the future of the NHS in trade talks directly with Mr Trump.
But Sky News has been told the pair "did not encounter" each other.
Mr Johnson hosted Ms Merkel, Mr Macron and Turkish President Recep Tayyip Erdogan for talks on Syria and terrorism.
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They talked about “deepening” their countries economic and defence partnerships and agreed “all attacks against civilians in Syria… must stop”, according to a Downing Street spokesperson.
The prime minister tried to play peacemaker, but that did not stop Mr Macron insisting he stood by his description of NATO as suffering from strategic "brain death".
Mr Trump sprung to NATOs defence, calling the comments "very, very nasty".
Mr Macron then suggested the organisation become less dependent on the US, calling for “more European defence to get … a less unbalanced NATO".
At the start of the day, the US president vowed to stay out of the general election – before quickly confirming “Im a fan of Brexit” and saying Mr Johnson was “very capable” and would “do a good job”.
Outside on the streets of London, hundreds of protesters marched with signs saying "no to racism, no to Trump", "lock him up" and "hands off our NHS".
The government says the health service will not be on the table when it comes to any trade deal struck with the US.