WASHINGTON — President Trump touched off another racially charged furor on Wednesday by sharing videos from a fringe British ultranationalist party purportedly showing Muslims committing acts of violence, a move that was swiftly condemned by Britain's prime minister as well as politicians across the spectrum.

Mr. Trump retweeted the video posts from an ultranationalist British party leader, Jayda Fransen, who has been charged in the United Kingdom with "religious aggravated harassment." The videos were titled: "Muslim migrant beats up Dutch boy on crutches!""Muslim Destroys a Statue of Virgin Mary!" and "Islamist mob pushes teenage boy off roof and beats him to death!"

At least one of the videos, however, did not show a "Muslim migrant," as it claimed, but a teenage boy who was born in the Netherlands, according to Dutch authorities. The other two showed incidents in Syria and Egypt in 2013 without any explanation of the context of the political unrest then taking place in those countries.

No modern American president has promoted inflammatory content of this sort from an extremist organization. Mr. Trump's two most recent predecessors, George W. Bush and Barack Obama, both made a point of avoiding public messages that were likely to be seen as anti-Muslim and could exacerbate racial and religious animosities, arguing that the war against terrorism was not a war against Islam.

But Mr. Trump has shown little such restraint, targeting Muslims with a broad brush, such as when he claimed on the campaign trail lastyear that "Islam hates us" and when he called for a "total and complete shutdown" of Muslims coming to the United States. Since taking office, he has sought to block visitors from select Muslim-majority nations and engaged in a long-distance feud with the Muslim mayor of London, whom he accused of being weak on terrorism.

Such unbridled talk has thrilled some of his supporters who see him as a truthteller breaking out of the shackles of political correctness, but it has alarmed mainstream political leaders both in the United States and Britain, who see it as reckless and counterproductive.

Senator Jeff Flake, a Republican of Arizona who has broken with Mr. Trump, called the postings "highly inappropriate" and added, "I hope he takes them down and doesn't do it again."

Senator Lindsey Graham, a South Carolina Republican who lately has been an ally of the president, said Mr. Trump was "legitimizing religious bigotry" with the tweets. "We need Muslim allies in the war on terror," he said. "I can only imagine how some of our Muslim allies must feel when the president gives legitimacy to it."

The reaction was sharp in London, where Prime Minister Theresa May, leader of the Conservative Party, denounced the president for embracing Britain First, the far-right party of Ms. Fransen. "It is wrong for the president to have done this," Ms. May's office said in a statement. "Britain First seeks to divide communities by their use of hateful narratives that peddle lies and stoke tensions. They cause anxiety to law-abiding people."

David Lammy, a member of Parliament for the Labour Party, echoed that statement on Twitter. "Trump sharing Britain First," he wrote. "Let that sink in. The President of the United States is promoting a fascist, racist, extremist hate group whose leaders have been arrested and convicted. He is no ally or friend of ours."

This reaction is exactly what James R. Clapper, the former director of national intelligence, said he feared when he saw the president's Twitter posts.

"It has all kinds of ripple effects, both in terms of perhaps inciting or encouraging anti-Muslim violence, and as well causes, I think, our friends and allies around the world to wonder about the judgment of the president of the United States," Mr. Clapper told CNN on Wednesday.

The White House spokeswoman, Sarah Huckabee Sanders, defended the president's tweets, saying he was talking about the need for national security and military spending.

"The threat is real," Ms. Sanders told reporters. "The threat needs to be addressed. The threat has to be talked about, and that's what the president is doing in bringing that up."

The first video distributed by Mr. Trump to his nearly 44 million followers on Wednesday showed a teenage boy attacking another and was presented as footage of a "Muslim migrant" beating a Dutch boy.

But according to local officials, both boys are Dutch. The clip was taken in Monnickendam, a small town in the North Holland province of the Netherlands in May and shows a teenager punching and kicking another boy holding a crutch, while a third person films them. Marleen van Fessem, a press officer for the public prosecutor's office of the North Holland province, confirmed the 16-year-old boy who was arrested after the video came to light was "born and raised in the Netherlands."

The two other videos — one taken in Syria in 2013 and another in Egypt in 2013 — are provided with no explanation of the political turmoil taking place in those countries at the time, and without details on the extremist affiliations of one of the men in the video.

Britain First was co-founded in 2011 by James Dowson, a far-right activist who later supported Mr. Trump during the 2016 presidential campaign and was part of the efforts to spread anti-Clinton news on social media. Mr. Dowson left the group in 2014, according to Hope Not Hate, a British anti-racism group.

Ms. Fransen has been accused of using "threatening, abusing or insulting words or behavior" in speeches and leaflets at events this fall in England.

Ms. Fransen thanked Mr. Trump for promoting her message in a Twitter post on Wednesday.

Mr. Trump is not among Ms. Fransen's Twitter followers. But the president does follow a conservative commentator, Ann Coulter, who on Tuesday retweeted the video purporting to show a Muslim migrant beating a Dutch boy.

The official Twitter account of Britain First also wrote to its more than 24,000 followers on Wednesday morning about Mr. Trump's posts. "Donald Trump has just retweeted Britain First's deputy leader Jayda Fransen THREE times," the group wrote.

Britain First calls itself a "patriotic" political party but has been criticized by human rights groups as being a far-right extremist group that engages in activities calculated to bait Muslims.

Formed in 2011 by former members of the far-right British National Party, the group states on its Facebook page that its mission is to fight "the many injustices that are routinely inflicted on the British people" and to defend British culture against the excesses of left-wing liberalism and political correctness.

Chuka Umunna, a Labour Party member of Parliament, wrote on Twitter that an invitation for Mr. Trump to come to Britain for a state visit should be immediately withdrawn. "The US President is normalising hatred. If we don't call this out, we are going down a very dangerous road. His invite should be withdrawn," he wrote.

Craig M. Considine, a lecturer in the department of sociology at Rice University and the author of several books on Muslims in the West, said he saw Mr. Trump's tweets as an effort to stir up hatred and intolerance of Muslims in Western countries and build a case for driving them out. Mr. Trump was essentially promoting the "clash of civilizations" theory that the West and Islam are incompatible, Dr. Considine said.

"He's playing on this fear, whipping up the fear," Dr. Considine said. "It is completely reckless."

Mr. Trump's tweets were welcomed by a former Ku Klux Klan leader, David Duke, who wrote on Twitter: "Thank God for Trump! That's why we love him!"

Mr. Duke was also supportive of the president's defense of the white nationalist movement in August after a bloody protest in Charlottesville, Va.

At the time, Mr. Trump said that there were "very fine people on both sides" of the protest, equating counterprotesters with those who brandished swastikas, Confederate battle flags and anti-Semitic banners.

One man is charged with murder in the death of a protester, who was hit by a car that rammed into a crowd demonstrating against the right-wing rally.

New York Times, November 28, 2017.

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November 29, 2017