In a tweet on Friday, Mr Trump said he asked a general “how many will die?” about 10 minutes before ordering strikes on three sites in response to the downing of a US surveillance drone over the Strait of Hormuz.

“150 people, Sir” came the official’s reply, according to Mr Trump.

The President said he then decided the US response was disproportionate to “shooting down an unmanned drone” and ordered the military to stand down.

“I am in no hurry, our Military is rebuilt, new, and ready to go,” he wrote on Twitter.

But fears of an impending attack remain high as the two countries teeter on the brink of war.

Previously simmering tensions between Iran and the United States started to boil over after the Middle Eastern nation shot down an American drone on Thursday (US time).

It comes as a growing number of global airlines re-route flights to avoid Iranian airspace after American aviation authorities banned its carriers from the area.

Qantas last night confirmed it would adjust its travels to avoid fight paths over the Strait of Hormuz and Gulf of Oman.

The New York Times reports Mr Trump approved retaliatory military strikes against Iranian targets, such as radar and missile batteries,

It says the operation had started, planes were in the air and ships were in position when the abrupt order came to pull back.

The Pentagon says the drone, a $US110 million RQ-4A Global Hawk, was shot down by an Iranian surface-to-air missile system while operating in international airspace over the Strait of Hormuz, one of the world’s most important shipping routes.

“Iranian reports that the aircraft was over Iran are false,” said Navy Captain Bill Urban from US Central Command.

“This was an unprovoked attack on a US surveillance asset in international airspace.”

The Department of Defence has released a map showing where it alleges the attack took place.

Iran claims it fired on the “intruding American spy drone” after it violated Iranian airspace. Foreign Minister Javad Zarif posted images of its alleged flight path on Twitter, along with what he said were the exact coordinates of the attack.

Naturally, those coordinates do not match the location released by the United States.

“We’ve retrieved sections of the US military drone in our territorial waters where it was shot down,” Mr Zarif said.

“We don’t seek war, but will zealously defend our skies, land and waters. We’ll take this new aggression to the UN and show that the US is lying about international waters.”

Meanwhile, a top Iranian commander reportedly warned the country was “ready for war”.

Nothing Iran has said in the wake of the incident suggests the attack was anything other than intentional. But American leaders are seeking to de-escalate the situation.

Speaking to reporters during an Oval Office meeting with Canadian Prime Minister Justin Trudeau, Mr Trump said he believed Iran may have shot down the drone by accident.

“I think probably Iran made a mistake. I would imagine it was a general or somebody that made a mistake in shooting that drone down,” Mr Trump said.

“Fortunately that drone was unarmed. There was no man in it and it was just over international waters, clearly over international waters, but we didn’t have a man or woman in the drone.

“We had nobody in the drone. It would have made a big difference, let me tell you. It would have made a big, big difference.

“I have a feeling, and maybe I’m wrong and maybe I’m right, but I’m right a lot, I have a feeling that it was a mistake made by somebody that shouldn’t have been doing what they did. I think they made a mistake.”

“Are you saying you don’t think it was an intentional strike on the drone?” a reporter asked.

“I don’t know. I find it hard to believe it was intentional, if you want to know the truth. I think it could have been somebody who was loose and stupid that did it,” he said.

“We’ll be able to report back and you’ll understand exactly what happened. But it was a very foolish move. That I can tell you.”

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The President later hosted Congressional leaders, including Democrats Nancy Pelosi and Chuck Schumer, at the White House to brief them on the situation.

Afterwards, Mr Schumer told reporters he was worried Mr Trump might “bumble” his way into war with Iran.

“I told the President that these conflicts have a way of escalating. The President may not intend to go to war here, but we’re worried that he and the administration may bumble into a war,” he said.

“The Democratic position is that congressional approval must be required before funding any kind of conflict in Iran. One of the best ways to avoid bumbling into war, a war that nobody wants, is to have a robust, open debate and for Congress to have a real say. We learned that in the run-up to Iraq.”

Republican Senate leader Mitch McConnell merely said the administration was considering “measured responses” to Iran.

Democrats are perhaps more concerned about Mr Trump’s advisers than the President himself — particularly national security adviser John Bolton, who has repeatedly advocated using military force against Iran over the last decade.

“If you cross us, our allies or our partners, if you harm our citizens, if you continue to lie, cheat and deceive, yes, there will indeed be hell to pay,” he said in a speech last year.

“We are watching and we will come after you.”

Behind the scenes, the President is reportedly reluctant to spark a conflict overseas. Yesterday he denied that any of his advisers were pushing him towards war.

“No, not at all. In fact in many cases it’s the opposite,” Mr Trump said.

“Look, I said I want to get out of these endless wars. I campaigned on that. I want to get out.”

The abruptly rescinded order to retaliate against Iran shows just how close such a conflict could be.

Tensions between Iran and the United States were already escalating long before the drone was shot down.

In May last year, Mr Trump withdrew from the nuclear agreement his predecessor Barack Obama had struck with Iran. He had labelled it “the stupidest deal of all time” during the 2016 election campaign.

Several months later, he started to reimpose economic sanctions on Iran.

Then, in April, Mr Trump announced he would designate Iran’s Islamic Revolutionary Guards a foreign terrorist organisation, overruling opposition to the move from the Pentagon.

Last month, Mr Bolton said the administration was sending an aircraft carrier strike force to the Middle East, in an explicit warning to Iran.

“Any attack on United States interests or those of our allies will be met with unrelenting force,” Mr Bolton said.

May also saw Iran announce it was scaling back the commitments it made under the nuclear deal with Mr Obama, and would move to resume its high level uranium enrichment if other world powers failed to protect it from US sanctions.

In recent weeks, US defence officials have explicitly blamed Iran for attacks on oil tankers in the Gulf of Oman.

Which finally brings us to this week. On Monday, the Trump administration announced it was sending 1000 more troops to the region, along with greater military resources.

“The Iranian attacks (on tankers) validate the reliable, credible intelligence we have received on hostile behaviour by Iranian forces and their proxy groups that threaten United States personnel and interests across the region,” said acting Defence Secretary Patrick Shanahan.

The situation is increasingly volatile, and it seems likely neither side has fired its last shot.