Both outbursts drew widespread condemnation throughout the U.K. and beyond, and raised the question: Are U.S.-U.K. ties irreparably damaged?
Most analysts would say the relationship may be a bit bruised but intact.
"The core of the relationship between the U.S. and U.K. rests on its nuclear, intelligence and special forces cooperation — and is usually protected from the vagaries of politics," said Tim Oliver of LSE Ideas, the foreign policy think tank of the London School of Economics. "The U.S. and U.K. trust each other in ways we don’t trust anyone else in the world."
That said, the usually courteous diplomatic relationship between the two nations may be a bit frayed.
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After the terror attack on London Bridge and Borough Market, in which at least eight people died and 50 were wounded, Trump took to Twitter to describe Khan as "pathetic."
The president appeared to misinterpret Khan's comments that Londoners had "no reason to be alarmed" by the visible increase in armed police officers in the capital. Trump seemed to mistake the mayor’s words as a sign that he was soft on terrorism.
At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!"
"I don’t think we should roll out the red carpet to the president of the USA in the circumstances where his policies go against everything we stand for,” Khan told British broadcaster Channel 4.
The spat was the latest example of tension between the U.S. and U.K. since Trump took office. In March, British intelligence agency GCHQ issued an angry denial of "ridiculous" comments Trump made suggesting that they spied on him at the behest of President Barack Obama.
Meanwhile, May expressed regret over Trump’s decision to withdraw the U.S. from the Paris Climate Agreement, although she notably didn’t join European leaders’ joint statement in support of the agreement.
"Trump’s vision is not resonating across Europe"
Less than a week before the election, May’s delay in defending London’s mayor did little to help her campaign, experts say.
"It is possible and even likely that some voters turned away from Theresa May as a result of her hand-holding with Donald Trump in the final days of the campaign," said Brian Klaas, a fellow in Global and Comparative Politics at the London School of Economics. "Trump's decision to attack London Mayor Sadiq Khan in the immediate aftermath of the London Bridge terror attack was seen, across party lines, as a disgraceful move."
"Brits and Americans should value the special relationship — it is essential for strategic interests and shared values on both sides of the Atlantic," Klaas added. "But that relationship is under even more strain as the next government will be so fragile that it cannot afford to take on political damage from Trump."
May’s inability to secure a majority in the House of Commons for her Conservative Party was a shock to many in the U.K., where her party held an approximate 20-point lead in the polls over the rival Labour Party when she called the election in April.
May's Conservatives retained the most seats — but she looked set to join forces with ultra-conservative lawmakers from Northern Ireland's Democratic Unionist Party to form a minority government.
"I don’t think that the relationship between the U.S. and U.K. was the key driver of the election by any stretch, but it’s clear that it didn’t play to May’s advantage," said Leslie Vinjamuri, an associate fellow for the U.S. and Americas program at Chatham House, a London-based think tank.
"The contours of this election demonstrate that the youth care about globalization and liberalism, things that Trump has distanced himself from," Vinjamuri added. "Trump’s vision is not resonating across Europe. It suggests that there is support in the U.K. for having close relationship with Europe and being international player, issues that are challenged in current era of American politics."
Rachel Elbaum is a London-based editor, producer and writer.