Nobody forced the crisis in Minneapolis upon President Trump. He chose to inflame the tinderbox himself when he issued an ultimatum to people protesting the death of a black man there under the custody of a white police officer.

In a pair of tweets sent at 12:53 a.m. Friday, Trump threatened to deploy the National Guard to use lethal force against demonstrators he denigrated as “THUGS.” His ominous warning — “when the looting starts, the shooting starts. Thank you!” — was flagged by Twitter as a violation of the social media platform’s rule against glorifying violence.

Having contributed to another national cleavage over racial justice, a president who was elected to lead the nation through crises effectively retreated from the responsibility of doing so on this one.

At the same time, Trump on Friday abdicated the traditional role of an American president abroad, ceding global leadership by announcing that he was “terminating” U.S. membership in the World Health Organization.

A couple of hours later, however, Trump decided he wanted to weigh in on Minneapolis after all, calling the events there “terrible” and encouraging peaceful protesters but warning against “lawless anarchy and chaos.”

Still, Trump’s silence earlier left a mark. Three and a half years into his most unorthodox presidency, Trump proved Friday that he still had the capacity to surprise with his dual withdrawals.

“He’s perfectly incapable of exercising leadership because he doesn’t understand what leadership is,” said Max Skidmore, a political scientist at the University of Missouri at Kansas City and author of a book on presidential leadership during health crises. “He thinks of leadership as whipping up outrage from a crowd, and having them yell and support him.”

Trump called an afternoon news conference in the Rose Garden, read a scripted statement railing against China and the WHO over the coronavirus pandemic, and then turned his back on journalists shouting questions about the unrest in Minneapolis. He neither issued a perfunctory call for unity nor talked about George Floyd, who died Monday after a police officer pinned him to the ground and knelt on his neck, prompting the simple plea, “I can’t breathe.”

“It’s a silence that speaks volumes,” said Eddie S. Glaude Jr., chairman of Princeton University’s African American studies department. “What his silence reveals is what he cares about and what he doesn’t, and we can only use that as evidence that he really doesn’t give a damn. . . . In so many ways, Donald Trump represents the death rattle of an old America, and it’s loud and it’s violent.”

Trump’s “when the looting starts, the shooting starts” phrase has an ugly racial past. The phrase was notoriously used in 1967 by Miami’s tough-talking police chief, Walter Headley, who was white, to warn robbers in the city’s black neighborhoods that he could use shotguns and dogs at his command.

Pressed by reporters, Trump claimed ignorance of the origins. “I’ve heard that phrase for a long time,” he said. “I don’t know where it came from or where it originated. . . . I wouldn’t know a thing like that.”

Trump earlier sought to explain his “shooting starts” comment with an awkwardly constructed pair of tweets Friday afternoon claiming that he meant to convey that looting often can lead to shooting. “It was spoken as a fact, not as a statement,” Trump wrote. “It’s very simple, nobody should have any problem with this other than the haters, and those looking to cause trouble on social media. Honor the memory of George Floyd!”

Trump did not elaborate, but his aides later defended his conduct.

“We have witnessed again the media’s relentless twisting of President Trump’s words, and the Democrats seizing on that, to take the entire nation down the worst road imaginable,” Trump reelection campaign manager Brad Parscale said in a statement. “Twitter also played a role by mislabeling the President’s tweet and fueling the misinformation. . . . Their behavior is reprehensible and should be roundly condemned by all Americans.”

Both Trump’s predecessor, President Barack Obama, and the Democrat vying to unseat and succeed him, former vice president Joe Biden, weighed in on Minneapolis.

Obama drew a connection between Floyd’s death and the everyday injustices facing black Americans, including the viral videotaped confrontation between a white woman and a black birdwatcher in Manhattan's Central Park earlier this week.

“We have to remember that for millions of Americans, being treated differently on account of race is tragically, painfully, maddeningly ‘normal,’ ” Obama wrote in a statement. He added: “This shouldn’t be ‘normal’ in 2020 America. It can’t be ‘normal.’ If we want our children to grow up in a nation that lives up to its highest ideals, we can and must be better.”

Biden, who launched his presidential campaign by condemning Trump’s equivocations in the wake of the deadly white supremacist rally in 2017 in Charlottesville, delivered a speech Friday renewing his call for a restoration of “the soul of America.”

“The original sin of this country still stains our nation today,” Biden said. “Sometimes we manage to overlook it, and just push forward with the thousand other tasks of daily life. But it’s always there. And weeks like this, we see it plainly. We are a country with an open wound. None of us can turn away. None of us can be silent.”

At the end of the day, after ducking the issue in the Rose Garden event, Trump delivered brief remarks during a meeting with business leaders in the State Dining Room.

“I want to express our nation’s deepest condolences and most heartfelt sympathies to the family of George Floyd. Terrible event — terrible, terrible thing that happened,” Trump said, adding that he has asked the Justice Department to expedite its investigation into Floyd’s death. “It should never be allowed to happen, a thing like that. But we’re determined that justice be served.”

Trump added: “We can’t allow a situation like happened in Minneapolis to descend further into lawless anarchy and chaos. . . . The looters should not be allowed to drown out the voices of so many peaceful protesters.”

Trump’s response Friday represented a departure from how he handled other recent acts of protest. When crowds of mostly white people, some armed, demonstrated outside the Michigan State Capitol in Lansing and demanded that Gov. Gretchen Whitmer (D) loosen regulations designed to slow the spread of the coronavirus, the president praised them.

“The Governor of Michigan should give a little, and put out the fire,” Trump tweeted on May 1. “These are very good people, but they are angry. They want their lives back again, safely! See them, talk to them, make a deal.”

But as footage of black demonstrators outside a police station that had been set ablaze in Minneapolis aired on national television late Thursday night and into the morning Friday, Trump adopted a different tone.

“Either the very weak Radical Left Mayor, Jacob Frey, get his act together and bring the City under control, or I will send in the National Guard & get the job done right,” Trump tweeted. “These THUGS are dishonoring the memory of George Floyd, and I won’t let that happen. Just spoke to Gov. Tim Walz and told him the Military is with him all the way. Any difficulty and we will assume control, but when the looting starts, the shooting starts. Thank you!”

Walz, a Democrat, told reporters that Trump’s tweets risked “inflaming” an already tense situation.

“It’s just not helpful,” Walz said. “The city of Minneapolis is doing everything they can. If mistakes are made and there’s an accountability, we need to do that. But, in the moment where we’re at, in a moment that is so volatile, anything we do to add fuel to that fire is really, really challenging.”

Seung Min Kim and Michael S. Rosenwald contributed to this report.