WASHINGTON – President Donald Trump abruptly canceled an attack on Iran on Thursday evening after the nation shot down a U.S. drone, he confirmed in a Friday tweet.

"We were cocked & loaded to retaliate last night on 3 different sights when I asked, how many will die," Trump said, adding: "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."

Trump added that he is in "no hurry" to respond to Iran because "our Military is rebuilt, new, and ready to go, by far the best in the world. Sanctions are biting & more added last night."

Trump's tweets confirmed accounts of what officials had said overnight. The estimate of 150 casualties came from military planners, according to a U.S. Defense official briefed on the matter but not authorized to speak publicly. There was no indication that Trump planned to reverse course and order .

The president's decision followed a day of high tensions in Washington as he and military officials weighed their options for retaliating against Iran. The nation claimed it shot the drone because it violated Iranian airspace, a claim the U.S. said is false.

The Pentagon called the action a "dangerous and escalatory attack," and Trump labeled it a "very big mistake" on Thursday, suggesting a retaliatory U.S. strike was imminent.

White House spokesman Hogan Gidley said Friday that Trump spoke with Saudi Arabian Crown Prince Mohammad bin Salman Al Saud about "the threat posed by the Iranian regime’s escalatory behavior."

House Speaker Nancy Pelosi, D-Calif., said the U.S. must calibrate a response to Iran that reduces conflict in the region while advancing American interests. She said she wasn't given a "heads up" about the potential for a mission Thursday night. Lawmakers who met Thursday with Trump stressed the importance of not bolstering hard-liners in Iran, she said.

"It’s a very dangerous country and there are divisions within the country about hardliners versus others," Pelosi said Friday. "There was bipartisan consensus that we didn’t want to do anything that would strengthen the hand of hardliners in Iran because that just makes matters worse."

But the head of the House Republican Conference, Rep. Liz Cheney of Wyoming, said it could be “a very serious mistake” if Trump doesn’t respond to Iran with more than tweets. The third-ranking Republican leader said other adversaries around the world were watching how the U.S. responds.

“We simply can’t allow America’s adversaries to think that they can shoot down a U.S. military drone with impunity,” Cheney told conservative radio host Hugh Hewitt on Friday. “The failure to respond to this kind of direct provocation that we’ve seen now from the Iranians, in particular over the last several weeks, could in fact be a very serious mistake.”

Other House Republican leaders – Minority Leader Kevin McCarthy of California, Armed Services Committee ranking member Mac Thornberry of Pennsylvania, Foreign Affairs Committee ranking member Michael McCaul of Texas and Intelligence Committee ranking member Devin Nunes of California – issued a joint statement supporting Trump.

"There must be a measured response to these actions," the lawmakers said, after shooting down the drone and attacks on two commercial oil tankers. "President Trump and his national security team remain clear-eyed on the situation and what must be done in response to increased Iranian aggression."

Thursday’s decision has echoes of one that came to dog his predecessor. In August, 2012, President Barack Obama issued Syrian President Basha al Assad an ultimatum: use chemical weapons and face the consequences.

“We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized,” Obama told reporters. “That would change my calculus. That would change my equation.”

In April 2013, Defense Secretary Chuck Hagel told reporters traveling with him in the Middle East that the administration believed Assad had been using chemical weapons. That prompted Sen. John McCain, R-Ariz., to remark acidly that Obama’s red line was “apparently written in disappearing ink.”

Five months later, in August 2013, Assad did use chemical weapons on civilians, killing more than 1,400 people, including hundreds of children. Obama sought approval from Congress to mount a military response but the measure in support never received a vote. Meantime, Syria agreed to a deal brokered with the U.S. and Russia to dismantle its chemical weapons program. Much of Syria’s stockpile of nerve agents was destroyed under international supervision, but Assad’s regime continued to mount smaller-scale attacks on civilian targets with chlorine gas.

By 2017, it became clear Syria hadn’t destroyed all of its more sophisticated chemical weapons after it launched with the nerve agent sarin. That prompted the first of two missile attacks ordered by President Trump against Syria.

Trump started taking criticism for his decision on Friday, including from Republicans.

Rep. Adam KIngzinger, an Illinois Republican and Iraq War veteran who sits on the House Foreign Affairs Committee, said the aborted strike “has the potential to invite the look of weakness” if the president doesn’t follow up with action, he said on MSNBC.

“The question now is: is there a response or is this the end of that chapter?’ And if this is the end of that chapter, I think it has some real potential negative consequences,” Kinzinger said. “This is a position where it shouldn't be partisan. America has to be clear that we will defend ourselves, and this is a case that I think (merits) that.”

Kinzinger, referencing Obama's decision on Syria, again pressed a response to the drone shooting.

"America is facing a crisis in confidence. Obama's decision to cancel a strike in Syria in 2013 has had ramifications that are still being felt today. We cannot let the provocations and attacks by Iran go unanswered," he said in a tweet.

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On Thursday, acting Defense Secretary Patrick Shanahan huddled Thursday with national security officials at the White House to discuss military options, including an attack on the site that launched the missile that destroyed the unmanned surveillance aircraft, according to a Defense official familiar with the discussions but not authorized to speak about them publicly.

Senior Pentagon officials met into the evening, preparing for what appeared to be imminent action against Iran. The targets likely to be struck: missile batteries that launched the attack against the drone, and the radars that tracked it.

Trump also invited congressional leaders for a meeting at the White House. Earlier Thursday, the White House summoned leaders in Congress for a late-afternoon briefing on Iran.

In recent weeks, the Pentagon has shifted an aircraft carrier strike group, B-52 bombers and thousands of troops to the region to confront Iran and protect American forces in the region. The bombers offer the option of launching long-range cruise missiles far from Iran's air defenses.

In the last two years, U.S. and allied forces have mounted limited airstrikes in Syria. In April 2018, American, British and French forces fired 105 missiles on three Syrian chemical weapons facilities a week after the regime of Bashar al Assad had launched a brutal attack that killed 40 civilians. A year earlier, Tomahawk cruise missiles struck an airfield in Syria where Pentagon officials said a chemical attack had originated.

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The Navy RQ-4 was flying in international airspace over waters where two tankers were attacked in recent weeks when it was blasted by a surface-to-air missile fired from a battery near Goruk, Iran, according to Air Force Lt. Gen. Joseph Guastella, commander of Air Force assets in the Middle East. Iran has denied attacking the tankers, and said Thursday that the drone had violated its airspace.

"This dangerous and escalatory attack was irresponsible and occurred in the vicinity of established air corridors between Dubai, United Arab Emirates, and Muscat Oman, possibly endangering innocent civilians," Guastella said.

Tensions in the region prompted the Federal Aviation Administration to prohibit U.S. airlines from flying over the Persian Gulf or the Gulf of Oman “until further notice” because of the heightened military activities and political disputes between the U.S. and Iran.

The FAA issued the warning late Thursday for passenger aircraft after a U.S. military drone was shot down in the region earlier in the day. The FAA said the drone was operating in the vicinity of airline routes above the Gulf of Oman.

The warning comes the same week that the Netherlands charged four suspects – three Russians and a Ukrainian – with shooting down Malaysian Airlines flight 17 above Ukraine in July 2014 with a Russian-made surface-to-air missile, killing 298 people. The incident happened as Ukrainian and pro-Russian rebels were fighting in the area.

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