What are the potential risks, he asked. How many people could be killed? What could go wrong?

Trump had already been briefed in detail on such questions earlier in the day, including a Pentagon estimate of up to 150 Iranian casualties. But — with national security adviser John Bolton joining the debate arguing strenuously in favor of the strikes — the president was asking about them again.

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By the time the meeting was over, Trump had given the order to stand down.

“We were cocked & loaded to retaliate last night on 3 different sights when I asked, how many will die,” Trump wrote Friday on Twitter, embellishing several of the events in question. “150 people, sir, was the answer from a General. 10 minutes before the strike I stopped it.”

Trump’s reversal was the culmination of 24 frenzied and frenetic hours marked by public inconsistencies, congressional bewilderment and palpable concern among national security experts that the administration might inadvertently stumble into the kind of bloody Middle East conflict that the president had campaigned so vigorously against.

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This account of how Trump approved and then abruptly canceled a military operation against Iran is based on interviews with more than 25 White House officials, lawmakers, congressional aides, military officials and others familiar with the process. Most spoke on the condition of anonymity to discuss internal deliberations.

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Just hours before the Oval Office meeting, the military had begun to mobilize around the mission of exacting a price from the regime that had shot one of its unmanned assets out of the air. The downing of the drone — which the United States says was flying in international airspace and Iran says had breached Iranian territory — was the latest in a series of events that have heightened tensions between the two nations.

The administration had already announced plans to move hundreds of troops into the region and had accused Iran of attacking tankers and taking other aggressive actions.

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The administration’s debate over how to respond to the drone attack played out over the course of four meetings at the White House on Thursday beginning at 7 a.m. Trump was supportive of military action throughout the day as Pentagon officials, lawmakers and national security officials made their cases for or against escalation, making his sudden change of mind all the more striking to those inside the administration.

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“The president is full of surprises,” said one senior administration official, describing the day.

After the United States had confirmed the loss of the drone, Bolton convened a group of top officials to discuss a potential response over breakfast, officials said.

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There was no need to determine whether Iran was behind the action: Iran’s Islamic Revolutionary Guard Corps had quickly claimed responsibility for shooting down the Navy RQ-4 Global Hawk.

Trump’s initial public response was a tweet at 10:15 a.m. declaring that “Iran made a very big mistake!”

Later in the morning, Trump gave the green light for the Pentagon to prepare for strikes, after he was informed it would take military officials several hours to get everything in place. He had been briefed about casualties and other risks before giving the approval, officials said.

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During an 11 a.m. meeting, attended by outgoing acting defense secretary Patrick Shanahan, incoming acting defense secretary Mark T. Esper and other top military brass, administration officials reviewed several different options for how to respond.

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A senior U.S. defense official, speaking on the condition of anonymity because of the sensitivity of the discussions, said Friday morning that the Pentagon had Navy assets poised to strike in Iran if directed, including ships accompanying the aircraft carrier USS Abraham Lincoln. Those attacks could have included airstrikes by jets or — more likely — Tomahawk missiles, the official said.

Among the ships that could have been involved were the guided-missile cruiser USS Leyte Gulf and the destroyer USS Bainbridge, the official said. Both vessels can carry Tomahawk missiles.

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Throughout the day of harried White House deliberations, congressional leaders of both parties were given only a cursory understanding of what Trump was considering.

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Around 11 a.m., lawmakers on Capitol Hill were being briefed by intelligence officials about the Iranian threat, a meeting that was scheduled before the drone shooting. It quickly morphed into a briefing aimed at getting lawmakers and their staffers up to date on the overnight events, congressional aides said.

During the briefing, congressional staffers received word that the White House had called a 3 p.m. meeting, just for senior leaders, focused on Iran. The timing set off alarm bells: Lawmakers were anticipating something serious, and soon, according to aides for several members who attended both briefings.

Trump added to the confusion during public comments as he met with Canadian Prime Minister Justin Trudeau. He at once suggested that Iran should be held accountable for what occurred and also appeared to minimize the significance of the incident because the aircraft was unmanned and its downing had not led to U.S. casualties. He said he believed it to be a mistake by a lower-level Iranian official.

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“I find it hard to believe it was intentional, if you want to know the truth,” Trump told reporters just after noon in the Oval Office. “I think that it could have been somebody who was loose and stupid that did it.”

As congressional leaders headed to the White House for the afternoon briefing, they were not sure what to expect. According to one senior congressional aide, some were bracing for Trump to tell members that an attack on Iran was underway.

Instead, what lawmakers got when they arrived in the Situation Room was a “dialogue,” according to House Intelligence Committee Chairman Rep. Adam B. Schiff (D-Calif.), who has often butted heads with the president.

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Schiff said Thursday that he thought the president was listening to lawmakers as they warned him of their concerns.

The White House had gathered top military and intelligence officials for the Situation Room briefing, including Shanahan, Esper and CIA Director Gina Haspel.

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Senate Majority Leader Mitch McConnell (R-Ky.) told reporters that the president was “engaged in what I would call measured responses.” According to several aides, Trump did not appear to endorse any specific course of action, and neither Democratic nor Republican leaders emerged from the meeting expecting imminent action.

Republican leaders in the House put out a joint statement in the evening, calling for a “measured response,” stating that the president and his staff were “clear-eyed on the situation,” and pledging to “stand ready to support our men and women in uniform.”

On what happened next, most lawmakers aren’t fully clear. Even as Trump was being approached by military leaders seeking a final sign-off for the strikes to begin Thursday evening, many top lawmakers remained in the dark.

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“No,” House Speaker Nancy Pelosi said Friday when asked whether she was notified by the White House about the planned strikes.

It would be customary for the White House to inform the House speaker and other congressional leaders of any military operation that had been approved.

Later in the afternoon, the Pentagon was gearing up to announce a strike. Around 6 p.m., defense officials including Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff and Deputy Defense Secretary David Norquist huddled with Shanahan in his office overlooking the Potomac River.

That’s when Trump started to ask key questions, as he huddled in the Oval Office with top advisers, including White House counsel Pat Cipollone.

The president was especially interested in the number of potential casualties, noting that Iran’s downing of a drone hadn’t killed any Americans. He was told again that as many as 150 Iranians could be killed.

“I thought about it for a second, and I said, ‘You know what? They shot down an unmanned drone, plane, whatever you want to call it. And here we are sitting with 150 dead people that would have taken place probably within a half an hour after I said go ahead.’ ” Trump said in an interview on NBC News’s “Meet the Press” on Friday. “And I didn’t like it . . . I didn’t think it was proportionate.”

U.S. officials said that the 150 figure was on the high end of a range of possibilities.

Trump’s decision to call off the strikes, citing the potential casualties, came as a surprise to Pentagon officials who had spent the late afternoon gearing up for the operation. Some Defense Department officials had privately expressed concerns because they saw it as a major escalation, officials said.

Trump’s cancellation at about 7 p.m. came about two hours before the strikes were supposed to occur. The decision stood in contrast to the hawkish policies advocated by some of Trump’s advisers, including Bolton, who has long supported regime change in Iran.

Before Trump made his ultimate decision, White House and National Security Council officials were bombarded by calls from allies and diplomats — though answers were scarce. Several aides left the White House around 6 p.m. believing a strike would be happening that evening, administration officials said.

It is unclear what the administration plans to do next. Trump, whose decision to withdraw the United States from the Iran nuclear deal has led to increased tensions, has said he is open to holding negotiations with Iran’s leaders. The president’s overtures have been publicly rebuffed by Tehran.

Trump told advisers Friday morning that he was glad he didn’t go forward with the strikes.

He also claimed on Twitter that new sanctions had been imposed on Iran on Thursday night. But officials said later Friday that no new measures had been imposed.

The whipsaw approach left some in Congress concerned that the president didn’t have a clear strategy for managing relations with Iran.

“During our meeting with the President at the White House, Congressional Leaders stressed the necessity that we work with our allies and not strengthen the hand of Iran’s hardliners,” Pelosi said in a statement. “Democratic Leaders emphasized that hostilities must not be initiated without the approval of Congress.”

Trump asked lawmakers and allies to go on television to defend him Friday, one aide said.

While some Republicans — including Senate Foreign Relations Committee Chairman James E. Risch of Idaho — voiced support for Trump’s ultimate decision, others were either silent or critical.

Rep. Adam Kinzinger (R-Ill.), a member of the House Foreign Affairs Committee, said Trump risked looking weak by backing down after supporting military action.

“There’s got to be a cost to Iran,” Kinzinger said Friday. “To shoot down a $200 million plane the size of an airliner that easily could have had 35 people on it? There needs to be response. So ultimately, am I disappointed today? Yes.”

Correction: An earlier version of this story gave an incorrect first name for incoming acting defense secretary Mark T. Esper. This story has been updated.