Trump arrives in London amid a flurry of tweets and a royal welcome

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.

Mr. 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 Mr. Trump as a statesman while, back home, the race to replace him – and talk of impeaching him – heated up. Yet Mr. 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, Mr. Trump unleashed a Twitter tirade after a newspaper column in which Mr. Khan said Mr. 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," Mr. Trump wrote just before landing. "He is a stone cold loser who should focus on crime in London, not me."

The president added that Mr. Khan reminded him of the "terrible" New York City Mayor Bill de Blasio, though "only half his height." Mr. de Blasio, a Democrat, is a longshot candidate in the 2020 presidential race. Khan supporters have previously accused Mr. 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 Mr. 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, Mr. 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.

But the U.S. 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 U.S. president has frayed.

A sense of deja vu quickly spread around London as Mr. Trump barreled into the visit.

A year ago, Mr. Trump also took aim at his hosts before landing on English soil, blasting Ms. May in an interview hours before she hosted him for dinner. Though he has spared Ms. May so far this time, he has praised her rival, prime ministerial hopeful Boris Johnson, just days before Ms. 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," Mr. 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 Mr. Trump's endorsement would hurt or help Mr. Johnson's chances of becoming prime minister. Mr. Trump said he may meet with Mr. Johnson this week.

Mr. Trump also told The Sunday Times that Britain should "walk away" from Brexit talks and refuse to pay a $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 Mr. Farage should be given a role in the Brexit negotiations.

After lunch with the queen, Mr. Trump was shown the collection at Buckingham Palace, where he inspected, among other items, an 18th-century map of New York, historic photos golf at St. Andrews, and books about birds and George Washington.

Mr. Trump will be honored later Monday at an extravagant state dinner at the Palace. Demonstrators are expected, including the possible return of a balloon depicting the president as a baby.

Even some of the pageantry could produce awkward moments. The formal tea hosted by Prince Charles brings together a future king who has warned repeatedly about the perils of climate change and a president who is actively dismantling U.S. policies designed to slow global warming.

In an interview with The Sun, Mr. 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 Mr. Trump in the past, prompting the president to tell the tabloid, "I didn't know that she was nasty."

Mr. Trump said later he thought Ms. Markle would be "very good" as a royal and claimed he only meant her comments were "nasty."

Mr. 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 Mr. 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 Mr. Trump, who has shown a willingness to inject partisanship into such moments.

This story was reported by The Associated Press. Additional reporting by Darlene Superville and Deb Riechmann in Washington and Gregory Katz in London.