PRESIDENT TRUMP’S visit to Britain this week represents a final miscalculation by the country’s hapless prime minister, Theresa May. Having repeatedly failed to win parliamentary approval for the terms for Britain’s departure from the European Union, deepening what has become the country’s worst political crisis since World War II, Ms. May was forced to announce her resignation last month. She nevertheless chose to press ahead with what promised to be a polarizing visit by Mr. Trump, whom she invited to become only the third U.S. president to be treated to a state visit.

Mr. Trump did not disappoint. He had hardly landed in London on Monday before he directed a stream of insults at the city’s mayor, Sadiq Khan, with whom he has previously feuded. He gave interviews to British newspapers blatantly interfering in London’s ongoing debates over Brexit and the contest to succeed Ms. May. For good measure, having watched a few minutes of CNN’s local broadcast, he suggested a boycott of AT&T, the cable network’s owner, as a way of forcing “big changes” in its coverage.

Ms. May described the visit as “an opportunity to further strengthen” the “special relationship” between Britain and the United States. In fact, it will serve to put on display the widening cracks Mr. Trump has introduced into one of the United States’ closest alliances. Hundreds of thousands of protesters are expected to cram central London Tuesday to reject the U.S. president, beneath a huge orange balloon portraying him as a baby in a diaper. They will be joined by leaders of the opposition Labour Party, who along with other prominent politicians boycotted the banquet for Mr. Trump hosted by Queen Elizabeth II.

Such substantive discussions as occur between Mr. Trump and Ms. May are likely to be contentious. The two governments are at odds about policy toward Iran, the use of telecommunications equipment from China’s Huawei and climate change, among other issues. In interviews with the British press, Mr. Trump offered ignorant and unhelpful advice about the Brexit impasse, suggesting that far right anti-E.U. campaigner Nigel Farage be dispatched to negotiate the new relationship or that Britain simply “walk away” from a deal with Brussels. Wading into the Conservative Party’s ongoing contest to succeed Ms. May, he praised former foreign secretary Boris Johnson, for the predictable reason that “he has been very positive about me and our country.”

Some in London predicted that praise from Mr. Trump would hurt rather than help Mr. Johnson and Mr. Farage, given the president’s enormous unpopularity; nearly 70 percent of Britons have a negative opinion of him. What’s clear is that the special relationship is under the same strain as other foundations of the Western liberal order buffeted by the Trump presidency. Most likely, it will survive, given the powerful cultural and economic bonds between the two countries and the enduring overlap of their security interests. But this week will be remembered as a low moment.

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