Video report by ITV News Washington Correspondent Robert Moore
Donald Trump has confirmed the US was "cocked and loaded" and ready to retaliate against Iran after it downed a US surveillance drone.
In a series of tweets, the US president said he was primed to order the military strike on three targets but stopped it with just ten minutes to spare when he learned that as many as 150 people would die.
He said: "We were cocked & loaded to retaliate last night on 3 different sights when I asked, how many will die.
"150 people, sir, was the answer from a General. 10 minutes before the strike I stopped it."
Mr Trump said the deaths would not be proportionate for losing an unmanned drone.
He added that Iran was much weaker now than when Barack Obama was in the White House and that "sanctions are biting".
Ominously, perhaps, he said he was "in no hurry" to hit back at Tehran.
On Friday morning, major world carriers, including British Airways, Australia's Qantas, Lufthansa of Germany and Dutch carrier KLM said they would not allow their planes to fly over the Strait of Hormuz following the drone shooting.
Abu Dhabi-based airline Etihad said it has "contingency plans" in place though stopped short of saying it was re-routing aircraft around the area.
The US Federal Aviation Administration had earlier barred all US-registered carriers from flying over the area.
The possibility of military action showed just how tense the situation is between Washington and Tehran as the Trump administration combines a "maximum pressure" campaign of economic sanctions with a buildup of American forces in the region.
According to an official, the strikes were recommended by the Pentagon and were among the options presented to senior administration figures.
The military operation was called off around 7.30pm Washington time (12.30am BST), after Mr Trump had spent most of Thursday discussing Iran strategy with top national security advisers and congressional leaders.
The downing of the US drone over the Strait of Hormuz prompted accusations from the US and Iran about who was the aggressor.
Iran insisted the drone violated Iranian airspace while Washington said it had been flying over international waters.
General Amir Ali Hajizadeh, head of the Revolutionary Guard's aerospace division, said they had warned "several times" before launching the missile.
He told state TV: "Unfortunately they did not answer."
He added Iran collected the debris from its territorial waters.
Mr Trump had earlier declared in a tweet that "Iran made a very big mistake!" - but he also suggested that shooting down the drone - which has a wingspan wider than a Boeing 737 - was a foolish error rather than an intentional escalation.
"I find it hard to believe it was intentional, if you want to know the truth," Mr Trump said at the White House.
"I think that it could have been somebody who was loose and stupid that did it."
He described the downing of the drone as "a new wrinkle ... a new fly in the ointment", but then added: "This country will not stand for it, that I can tell you."
He said the American drone was unarmed and unmanned and "clearly over international waters".
It would have "made a big, big difference" if someone had been inside, he said.
But fears of open conflict shadowed much of the discourse in Washington.
As the day wore on, top national security advisers and congressional leaders were summonsed to the White House for an hour-long briefing in the Situation Room.
Citing Iranian threats, the US recently sent an aircraft carrier to the Persian Gulf region and deployed additional troops alongside the tens of thousands already there.
"We do not have any intention for war with any country, but we are fully ready for war," Revolutionary Guard commander Gen. Hossein Salami said in a televised address.
Iranian Foreign Minister Mohammad Javad Zarif tweeted that his country had retrieved sections of the military drone "in OUR territorial waters where it was shot down".
He said, "We don't seek war but will zealously defend our skies, land & waters."