President Trump said the U.S. military was "cocked and loaded" to retaliate against Iran on three different sites Thursday night, but called off the operation after learning 150 people would likely die from the strike. Mr. Trump, explaining his decision on Twitter, said the response would have been "not proportionate to shooting down an unmanned drone."

Mr. Trump further explained his decision in an interview with NBC's Chuck Todd on Friday. The president said military leaders came to him about 30 minutes before the planned strike, when he said he wanted to know one thing before moving forward — how many people would be killed? Those officials returned to him and put the number at approximately 150 people. The president said that he thought about it and considered the human toll. Mr. Trump claimed U.S. aircraft weren't in the air yet, but would have been soon, when he called off the strike.

"And 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, and I didn't like it. I didn't think, I didn't think it was proportionate," Mr. Trump told Todd.

House Speaker Nancy Pelosi told reporters Friday that she received no heads up a strike was in the works.

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"I did not receive any heads up that that there was any strike in the works," she said, calling it a departure from previous White House practice.

As CBS News has reported, the president ordered the drone strike Thursday night, but backed down, avoiding — for now — a significant escalation of tensions between Iran and the U.S. The president originally explained his decision in tweets Friday morning.

"On Monday they shot down an unmanned drone flying in International Waters," Mr. Trump tweeted Friday morning, referencing Iran. "We were cocked & loaded to retaliate last night on 3 different sights [SIC] when I asked, how many will die. 150 people, sir, was the answer from a General. 10 minutes before the strike I stopped it, not....proportionate to shooting down an unmanned drone. I am in no hurry, our Military is rebuilt, new, and ready to go, by far the best in the world. Sanctions are biting & more added last night. Iran can NEVER have Nuclear Weapons, not against the USA, and not against the WORLD!"

It's unclear what sanctions the president was referring to, as the Treasury Department announced no new sanctions on Thursday.

As for which general answered the president's question about the number of casualties estimated for the strike, CBS News' David Martin says that it's likely to have been one of two advising Mr. Trump -- Joint Chiefs of Staff Chairman Gen. Joseph Dunford, and Gen. Frank McKenzie, the head of CENTCOM. McKenzie directs operations in the Persian Gulf.

The aborted operation comes at a time of transition for the Pentagon. Acting Defense Secretary Patrick Shanahan just withdrew from consideration to be the permanent defense secretary, but until Sunday, he'll still be on the job. Army Secretary Mark Esper has been named acting defense secretary, and both men attended meetings on the Iran operation.

Mr. Trump met with top congressional leaders at the White House Friday to decide how to respond to Iran shooting down the unmanned drone, an act the president said could be due to someone "loose and stupid" and an unintentional mistake. After that meeting, top Democrats agreed the situation with Iran is serious. But a source in the room told CBS News that no options for a response were posed at the time.

On Friday, Mr. Trump was scheduled to have his regular intelligence briefing behind closed doors at 11:30 a.m., meet with Secretary of State Mike Pompeo, and host members of Congress for a picnic on the White House South Lawn.

The president also spoke with Saudi Arabian Crown Prince Mohammad bin Salman about Iran on Friday, according to principal deputy press secretary Hogan Gidley.