Donald Trump has touched down in the UK ahead of a two-day NATO summit that will see members at loggerheads with each other.
He will join other NATO leaders in London on Tuesday as the world's biggest military alliance marks its 70th birthday.
Experts say that with arguments over military spending and NATO's purpose in the air, the 29 NATO allies are approaching this meeting "with a sense of foreboding".
Thousands of troops stand ready along Europe's eastern flank to deter President Vladimir Putin - but France's President Macron fears terrorism is the greatest threat to the group and Turkey is buying missiles from Russia.
The summit will include receptions at Buckingham Palace and Downing Street and a working session at a golf resort in outer London.
NATO Secretary General Jens Stoltenberg invited the UK to host this special anniversary meeting in recognition of the UK's significant role in the organisation's history.
Prime Minister Boris Johnson will try to play peacemaker by breaking off from the election campaign trail for this year's summit.
"The PM's position is that NATO is the most enduring and successful alliance in military history and that it continues to adapt to the evolving threats that we face," his spokesman said.
"It is the cornerstone of Euro-Atlantic security and it helps to keep a billion people safe.
"The PM will emphasise that all members must be united behind shared priorities so NATO can adapt to the challenges ahead."
A White House statement last month said: "President Trump looks forward to meeting with the other NATO Heads of State and government to review the Alliance's unprecedented progress on burden-sharing, including adding more than $100bn in new defence spending since 2016.
"The president will also emphasise the need for the NATO Alliance to ensure its readiness for the threats of tomorrow, including those emanating from cyberspace, those affecting our critical infrastructure and telecommunications networks, and those posed by terrorism."
Here is what is expected to happen over the coming days:
Mr Trump transferred from the airport and stayed overnight at the US Ambassador's residence in Regent's Park, Winfield House.
Daytime - Official outreach event called NATO Engages: Innovating the Alliance in central London. The NATO secretary general, deputy secretary general and other leaders are expected to speak. Ben Wallace will attend for the UK. Unknown if Mr Trump will attend.
Daytime - Preparations get under way for the NATO summit on Wednesday in Watford with several road closures, diversions in place and bans implemented on the use of drones.
3pm - Demonstrators start to assemble in Trafalgar Square before marching down The Mall, arriving at Canada Gate, opposite Buckingham Palace, at around 5.45pm, shortly before Mr Trump arrives at Buckingham Palace. Stop the War, CND, and other groups are expected to attend.
Evening - Reception for NATO Leaders to mark 70 years of the NATO Alliance at Buckingham Palace, hosted by the Queen.
Afterwards, there will be a further reception at 10 Downing Street, also attended by UK officials with connections to NATO and the winners of the NCSC CyberFirst Girls Competition from the Tiffin School.
From 7.30am - Leaders start arriving for the NATO Leaders Meeting at the Grove Hotel, Watford, Hertfordshire
9am - Official greeting by the Secretary General and the Prime Minister of the United Kingdom Boris Johnson. 'Family' photo.
10am - Working session of the North Atlantic Council at the level of Heads of State and Government. Leaders are expected to "reflect on the last 70 years and discuss issues affecting the Alliance including NATO missions, member states' spending and NATO's role in responding to threats around the world", according to Downing Street.
Members will also use the meeting to look ahead to new challenges, including in the areas of cyber and space.
Among those attending are: Edi Rama (Albanian PM), Sophie Wilmes (Belgian PM), Ruman Radev (Bulgarian President), Justin Trudeau (Canadian PM), Andrej Plenkovic (Croatian PM), Miloš Zeman (Czech President), Mette Frederiksen (Danish PM), Jueri Ratas (Estonian PM), Emmanuel Macron (French PM), Angela Merkel (German Chancellor), Kyriakos Mitsotakis (Greek PM), Viktor Orban (Hungarian PM), Katrin Jakobsdottir (Icelandic PM), Giuseppe Conte (Italian PM), Egils Levits (Latvian President), Gitanas Nauseda (Lithuanian President), Xavier Bettel (Luxembourger Prime Minister), Dusko Markovic (Montenegrin PM), Mark Rutte (Dutch PM), Zoran Zaev (North Macedonia PM), Ema Solberg (Norwegian PM), Andrzej Duda (Polish President), António Costa (Portuguese PM), Klaus Wener Iohannis (Romanian President), Zuzana Caputova (Slovakian President), Marjan Sarec (Slovenian PM), Pedro Sanchez (acting Spanish PM), Recep Tayyip Erdogan (Turkish President), Donald Trump (United States President).
1.30pm: News conference with the secretary general and other leaders.