WASHINGTON — On one level, US President Donald Trump reacted to Saturday (June 3) night’s terrorist attack in London much as his predecessors might have. He expressed solidarity and telephoned Britain’s prime minister to offer condolences. “WE ARE WITH YOU,” he wrote to Britons.

But even as the investigation into the attack was getting underway, Mr Trump wasted little time in using the episode to defend his hotly disputed travel ban on visitors from certain predominantly Muslim countries and to criticise the judges who have blocked it. And by Sunday morning, he decided to go after the mayor of London as not being tough enough on terrorism.

Along the way, he characterised the mayor’s position, renewed a trans-Atlantic feud stretching back a year and widened his rift with the United States’ traditional European allies a bit further. And he set off a chain reaction in the news media world, triggering partisan reactions that illustrated just how polarised both America and the world have become about the uninhibited, Twitter-obsessed president.

Mr Trump’s penchant for picking fights is well established by now, but it continues to confound and exasperate foreign leaders who are not accustomed to such rough-and-tumble interactions with US presidents. The niceties of international diplomacy have never had such a reality-show flavor to them in the modern era, but Mr Trump has thrilled his nationalist base with his “America First” approach, and all the complaints from overseas only seem to embolden him.

The latest contretemps came when Mr Trump heaped scorn on Mayor Sadiq Khan of London. “At least 7 dead and 48 wounded in terror attack and Mayor of London says there is — no reason to be alarmed!’” Mr Trump wrote on Twitter.

Mr Trump either misunderstood what Mr Khan had said or distorted it. During an interview shown on the BBC, the mayor said he was “appalled and furious that these cowardly terrorists would target” innocent civilians and vowed that “we will never let them win, nor will we allow them to cower our city.”

He went on to say that residents should not worry as they encounter more police officers patrolling the streets.

“Londoners will see an increased police presence today and over the course of the next few days,” Mr Khan said. “No reason to be alarmed. One of the things the police, all of us, need to do is make sure we’re as safe as we possibly can be. I’m reassured that we are one of the safest global cities in the world, if not the safest global city in the world, but we always evolve and review ways to make sure that we remain as safe as we possibly can.”

Mr Khan’s office later dismissed Mr Trump’s post, saying the mayor was too busy to reply. “He has more important things to do than respond to Donald Trump’s ill-informed tweet that deliberately takes out of context his remarks urging Londoners not to be alarmed when they saw more police — including armed officers — on the streets,” his office said in a statement.

The exchange reflected the tensions between Mr Trump and the United States’ traditional allies in Europe. The president returned home a little more than a week ago after meetings in Belgium and Italy that put on display disputes over trade and the role of NATO, then followed up by pulling the United States out of the Paris climate change accord, over the objections of European leaders.

The friction has been especially acute for more than a year between Mr Trump and Mr Khan, the first Muslim to serve as mayor of a major Western European capital. During last year’s presidential race, Mr Khan criticised Trump’s proposal to temporarily ban all Muslims from entering the United States and endorsed Hillary Clinton, prompting an exchange with Mr Trump’s campaign.

Critics of Mr Trump in Britain and the United States faulted him for his acrimonious response to the Saturday assault. “I don’t think that a major terrorist attack like this is the time to be divisive and to criticise a mayor who’s trying to organise his city’s response to this attack,” former Vice President Al Gore said Sunday on CNN’s “State of the Union.”

Mr Damon Wilson, who was President George W Bush’s top Europe adviser, lamented that the spat only undercut a relationship that mattered to the United States. “America is safer when we rally our friends and allies against the bad guys rather than pick fights with the good guys,” he said.

But the White House showed no signs of backing down, and a top aide to Mr Trump amplified the attack shortly after ward. Mr Dan Scavino Jr, the president’s director of social media, posted a message referring to Mr Khan’s criticism of Trump a year ago for his “ignorant view of Muslims”.

Addressing Mr Khan, Scavino referred to his “tweet 13 months ago, after you criticised ... now President @realDonaldTrump — and WAKE UP!!!!”

Other US officials sought to smooth over the dispute. “I commend the strong leadership of the @MayorofLondon as he leads the city forward after this heinous attack,” Mr Lew Lukens, a career diplomat serving as acting US ambassador to London, wrote on Twitter hours later.

This is just the latest time the US Embassy in London has had to manage turbulence in the relationship since Trump took office. The London government complained vociferously after the White House aired a conspiracy theory that British intelligence helped President Barack Obama secretly spy on Mr Trump during last year’s campaign, which Britain denied. London complained again when US officials leaked details of the investigation into last month’s terrorist bombing in Manchester.

Mr Trump’s initial arguments about the meaning of Saturday night’s terrorist attack stirred debate both at home and abroad.

“We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!” he wrote in one message Saturday night.

“We must stop being politically correct and get down to the business of security for our people,” he wrote in another on Sunday morning. “If we don’t get smart it will only get worse.”

“Do you notice we are not having a gun debate right now?” he added. “That’s because they used knives and a truck!”

Mr Trump first tussled with Khan publicly last year shortly after Khan was elected. At the time, Mr Trump’s proposed Muslim ban applied to all Muslims from all countries who were not US citizens, but Mr Trump said he welcomed Mr Khan’s election and would make an exception for him.

Mr Khan replied that he wanted no exception and accused MR Trump of harbouring “ignorant views about Islam.” Mr Khan said Trump would only encourage Muslim alienation from the West. “He’s playing into the hands of extremists,” he said.

Mr Scavino, at the time, fired back on Mr Trump’s behalf, writing on Twitter that it was not ignorance: “It’s called not being ‘politically correct.’ @realDonaldTrump will MAKE AMERICA SAFE & GREAT AGAIN!”

The feud continued after Mr Trump’s election. In March, after a terrorist attack in London, Mr Donald Trump Jr. posted a link suggesting that Khan had said terrorist attacks were just “part of living in big city.” In fact, what Khan had said was that “part and parcel of living in a great global city” was being vigilant against terrorism and supporting police. THE NEW YORK TIMES