Shortly before landing near London, the US president tweeted the city's mayor was a "stone-cold loser" after Sadiq Khan criticised the British government for offering a state visit to Donald Trump.
Mixing pageantry and political pugilism, President Donald Trump opened a state visit to Britain on Monday by drawing a smile from Queen Elizabeth II and stepping up a long-running feud with London's anti-Trump mayor before his plane had touched down on English soil.
Trump and his wife, Melania, flew by helicopter to Buckingham Palace, landing on a lawn where Prince Charles and his wife, Camilla, greeted them. They received a deafening royal gun salute as they walked to the palace where a waiting queen smiled at the president.
Those were the images sought by a White House eager to showcase Trump as a statesman while, back home, the race to replace him — and talk of impeaching him — heated up.
TRT World's Simon McGregor-Wood brings more from London.
Yet Trump, forever a counter-puncher, immediately roiled diplomatic docility by tearing into London Mayor Sadiq Khan.
With the trip already at risk of being overshadowed by Brexit turmoil, Trump unleashed a Twitter tirade after a newspaper column in which Khan said Trump did not deserve red-carpet treatment and was "one of the most egregious examples of a growing global threat" from the far-right to liberal democracy.
"@SadiqKhan, who by all accounts has done a terrible job as Mayor of London, has been foolishly "nasty" to the visiting President of the United States, by far the most important ally of the United Kingdom," Trump wrote just before landing.
"He is a stone cold loser who should focus on crime in London, not me."
.@SadiqKhan, who by all accounts has done a terrible job as Mayor of London, has been foolishly “nasty” to the visiting President of the United States, by far the most important ally of the United Kingdom. He is a stone cold loser who should focus on crime in London, not me......— Donald J. Trump (@realDonaldTrump) June 3, 2019
The president added that Khan reminded him of the "terrible" leader of his hometown, New York City Mayor Bill de Blasio though "only half his height." De Blasio, a Democrat, is a longshot candidate in the 2020 presidential race. Khan supporters have previously accused Trump of being racist against London's first Muslim mayor.
The president then added warm words for his hosts, tweeting that he looked forward "to being a great friend to the United Kingdom, and am looking very much forward to my visit."
The agenda for Trump's weeklong journey is mostly ceremonial: a state visit and audience with the queen, D-Day commemoration ceremonies on both sides of the English Channel and his first presidential visit to Ireland, which will include a stay at his coastal golf club.
During the palace welcome ceremony, Trump and Prince Charles inspected the Guard of Honor formed by the Grenadier Guards wearing the traditional bearskin hats.
Royal gun salutes were fired from nearby Green Park and from the Tower of London as part of the pageantry accompanying an official state visit, one of the highest honors Britain can bestow on a foreign leader.
The Queen, President Trump and the First Lady view a display of items from the Royal Collection illustrating the enduring relationship between the US and the UK. #USStateVisit pic.twitter.com/gPuI5Ziab6— The Royal Family (@RoyalFamily) June 3, 2019
But the US president arrived at a precarious moment.
He faces a fresh round of impeachment fervor back home and uncertainty on the other side of the Atlantic.
British Prime Minister Theresa May has faced months of political turmoil over Brexit and French President Emmanuel Macron is expected to use the 75th anniversary of the World War II battle that turned the tide on the Western Front to call for strengthening multinational ties the US president has frayed.
A sense of deja vu quickly spread around London as Trump barreled into the visit.
A year ago, Trump also took aim at his hosts before landing on English soil, blasting May in an interview hours before she hosted him for dinner. Though he has spared May so far this time, he has praised her rival, prime ministerial hopeful Boris Johnson, just days before May steps down as Conservative leader on Friday for failing to secure a Brexit deal.
"I think Boris would do a very good job.
I think he would be excellent," Trump told The Sun. "I like him. I have always liked him. I don't know that he is going to be chosen, but I think he is a very good guy, a very talented person."
It was not clear if the Trump endorsement would hurt or help Johnson's chances of becoming prime minister. Trump said he may meet with Johnson this week.
Never shy about weighing in on other countries' affairs, Trump also told the Sunday Times that Britain should "walk away" from Brexit talks and refuse to pay a 39 billion pound ($49 billion) divorce bill if it doesn't get better terms from the European Union.
He said he might meet with another pro-Brexit politician, Nigel Farage, and claimed Farage should be given a role in the Brexit negotiations.
After lunch with the queen, Trump was shown parts of the collection at Buckingham Palace, including an 18th-century map of New York, historic photos of golf at St. Andrews and books about birds and George Washington. Westminster Abbey was next, with a tour and moment of silence at the tomb of the Unknown Warrior.
He will be honored later on Monday at an extravagant state dinner at the Palace.
Even some of the pageantry could produce awkward moments. A formal tea hosted by Prince Charles brought together a future king who has warned repeatedly about the perils of climate change and a president who is actively dismantling US policies designed to slow global warming.
In an interview with The Sun, Trump weighed in on the American-born Duchess of Sussex. The former Meghan Markle, who gave birth to a son in May and will not attend the week's events, was critical of Trump in the past, prompting the president to tell the tabloid, "I didn't know that she was nasty."
Trump said later he thought Markle would be "very good" as a royal and claimed he only meant her comments were "nasty."
Londoners and tourists gathered by Parliament Square, many of them to protest US President Donald Trump's state visit
Protesters were seen holding up placards with anti-Trump messages and slogans, and one of them carried a minitature Trump "blimp" balloon.
A large-scale demonstration is expected in the UK capital on Tuesday, when thousands are planning to gather to voice their opposition against Trump's visit.
Trip to Ireland
Trump will also make his first presidential visit to Ireland on Wednesday, spending two nights at his golf club in Doonbeg, which sits above the Atlantic. After Dublin balked at holding a meeting there, a deal was struck for Trump to meet Irish Prime Minister Leo Varadkar at the VIP lounge at Shannon Airport, hardly the grand setting usually afforded a meeting of world leaders.
The centerpiece of the president's visit will be two days to mark the 75th anniversary of the June 6, 1944, D-Day anniversary, likely the last significant commemoration most veterans of the battle will see. The anniversary events will begin in Portsmouth, England, where the invasion was launched, and then move to Normandy, France, where Allied forces began to recapture Western Europe from the Nazis.
The day is normally a heartfelt tribute to unity and sacrifice, outweighing any national or political skirmish. But some on both sides of the Atlantic are nervous about Trump, who has shown a willingness to inject partisanship into such moments.