WASHINGTON • By the time President Donald Trump met congressional leaders on the afternoon of June 20, he had already decided to retaliate against Iran for shooting down a US surveillance drone. But for once, he kept his cards close to the vest, soliciting advice rather than doing all of the talking.

"Why don't you go after the launch sites?" a Republican lawmaker asked.

"Well," Mr Trump replied with a hint, "I think you'll like the decision."

But barely three hours later, Mr Trump had changed his mind. Without consulting his vice-president, secretary of state or national security adviser, he reversed himself and, with ships readying missiles and aeroplanes already in the skies, told the Pentagon to call off the air strikes with only 10 minutes to go.

When Vice-President Mike Pence and other officials returned to the White House for what they expected would be a long night of monitoring a military operation, they were stunned to learn the attack was off.

That about-face, so typically impulsive, instinctive and removed from any process, proved a decision point for a President who has often threatened to "totally destroy" enemies but at the same time has promised to extricate the US from Middle East wars.

It revealed a commander-in-chief more cautious than critics have assumed, yet underscored the limited options in a confrontation he had set in motion.

Three months later, some of Mr Trump's own allies fear the failure to follow through had been taken by Iran as a sign of weakness, emboldening it to attack oil facilities in Saudi Arabia this month.

Mr Trump argues his decision was an expression of long-overdue restraint by a nation that has wasted too many lives and dollars overseas. But he finds himself back where he was in June, wrestling with the consequences of using force and the consequences of avoiding it, except that now Iran is accused of an even more brazen provocation, and the stakes seem even higher.

As Mr Trump again weighs retaliation against Iran, this time for the Saudi attacks, the choice he made in June is instructive in the insight it provides into how he approaches a life-or-death decision committing US forces against an enemy.

This account of that day in June is based on interviews with White House aides, Pentagon officials, military officers, US and foreign diplomats, members of Congress and outside presidential advisers, most of whom asked not to be identified.

That day clearly stays with Mr Trump, who has ruminated on it over the past week.

"When I was running, everybody said, 'Oh, he's going to get into war, he's going to get into war, he's going to blow everybody up, he's going to get into war'," he told reporters last Friday. "Well, the easiest thing I can do - in fact, I could do it while you're here - would say, 'Go ahead, fellas, go do it'. And that would be a very bad day for Iran."

But as eager as he is to fight with 280 characters on Twitter, Mr Trump has proved profoundly reluctant to fight with live ammunition on a real battlefield. "For all of those that say, 'Oh, they should do it, it shows weakness', actually, in my opinion, it shows strength."

PROPORTIONAL RESPONSE

Mr Trump came to office fixated on Iran as an enemy to be confronted. In abandoning the nuclear agreement negotiated by then President Barack Obama in 2015 on the grounds that it was a bad deal, Mr Trump set himself on a collision course with Teheran that was bound to test him.

Strained by the "maximum pressure" sanctions that Mr Trump has imposed, Iran this summer acted out aggressively, targeting oil tankers in the Gulf of Oman and vowing to reconstitute its nuclear programme. The overnight downing of the Global Hawk drone in June seemed to be the climax of a campaign of escalation that would draw in Mr Trump.

Hours after the drone was destroyed, Mr Trump's team met for breakfast at 7am in the office of Mr John Bolton, then the national security adviser. Secretary of State Mike Pompeo and General Joseph Dunford, the chairman of the Joint Chiefs of Staff, were joined by two acting Secretaries of Defence, Mr Patrick Shanahan, who had just announced his resignation and was days away from departing, and Dr Mark Esper, his replacement.

At the meeting, several strike options were discussed. The Pentagon's preferred plan was to attack one of the missile-laden Iranian boats that the US had been tracking in the Gulf of Oman. American forces would warn the Iranians to evacuate the vessel, videotape them doing so, then sink the boat with a bomb or missile strike.

The end result would be zero casualties, which Mr Shanahan and Gen Dunford argued would be a proportional response to the downing of a US$130 million (S$179 million) drone that had itself resulted in no loss of life.

Mr Bolton and Mr Pompeo were concerned that would not be decisive enough and pushed for strikes on Iranian soil. Mr Bolton argued for what was described as a "comprehensive list" of targets, but only so many could be hit if the operation was to be carried out quickly, so the officials settled on three Iranian missile batteries and radars.

The same advisers reconvened along with more officials at 11am in the Situation Room to brief the President. The meeting lasted for about an hour as various possibilities were discussed.

Four officials said that striking the three targets would result in about 150 casualties, a number derived from Iranian manning doctrine for these particular facilities, including operators, maintenance personnel and security guards.

How much Mr Trump was paying attention to that part of the briefing or what he absorbed was not clear in hindsight to some officials.

RE-ELECTION IN MIND

The national security team emerged from that meeting convinced it had a decision from Mr Trump to strike, and soon the aircraft carrier Abraham Lincoln and other ships and aircraft were on the move, preparing for an attack at around 9pm Washington time, or just before dawn in the region.

Still, there continued to be pushback from Pentagon civilians and Gen Dunford. They argued that killing up to 150 Iranians did not equate to the shooting down of a drone and could prompt a counter-strike by Iran that would escalate into a broader confrontation.

Moreover, Gen Dunford argued that a sustained conflict in the Middle East would require the US to divert more forces to the region, including from the Pacific theatre, which would benefit China.

Mr Trump seemed to be nursing doubts of his own, partly because of reports that the Iranian commander who shot down the drone had acted on his own, not on specific orders of the national government.

Mr Bolton argued that it mattered little if Teheran gave the order or gave its commanders so much authority that they could take such action on their own. But it clearly did matter to Mr Trump.

It also mattered that he had been so critical of past presidents for being too quick to pull the trigger. In the days leading up to this moment, he had talked with Mr Tucker Carlson, the Fox News host, who reminded him that he had come to office to get out of endless wars, not start a new one.

If he allowed himself to be pulled into a new conflict by the same people who got the US into Iraq, then Mr Trump could forget about his chances for re-election, Mr Carlson told him.

WORRIED

At 3pm, Mr Trump convened a dozen congressional leaders from both parties in the Situation Room, a rare act of inclusion. Mr Pence, Mr Pompeo, Mr Shanahan, Dr Esper, Mr Bolton and Mr Pat Cipollone, the White House counsel, joined the meeting.

Mr Trump rambled on about how bad Mr Obama's deal had been and insisted over and over again - one lawmaker estimated a dozen times - that his pressure campaign would force Iran to the bargaining table.

He seemed less certain about what to do in response to the drone shoot down. Democrats suggested caution, warning that a military strike could destabilise the region and play into Iran's hands.

Mr Trump, for once, did not reject their views. "At the end of the day, the impression I got was that the President was genuinely worried about stumbling into a broader conflict," said Representative Adam Smith, chairman of the House Armed Services Committee.

'THE ORDER DIDN'T COME'

As the hour approached, Mr Trump was given the estimate that 150 Iranians would be killed in the attack. The President later said publicly that it came when he asked his generals, but in fact, multiple officials said the estimate was delivered to him by a White House lawyer who got it from a Pentagon lawyer.

Pentagon lawyers typically prepare casualty estimates drawn from manuals listing how many personnel are believed to work at certain foreign facilities.

Advocates of the strike complained that the estimate was just a formula that did not account for the fact that the attack would be conducted at night when few if any personnel would be on duty.

Why Mr Trump suddenly latched onto the estimate at this point rather than when casualties were discussed at the earlier meeting remains a mystery to many officials.

Mr Pence, Mr Pompeo and Mr Bolton were all out of the White House when Mr Trump told the Pentagon to call off the attack.

Gen Dunford called General Kenneth McKenzie, the head of the Central Command in Tampa, Florida, with new orders from the White House: The strike was off. The Tomahawk missiles stood down. Attack planes were called back.

In the command centre of the Abraham Lincoln, Rear-Admiral Michael Boyle, the carrier strike group commander, had been waiting for the final order to attack.

"All the systems were on, all the lights were green, we were waiting for the order," he recalled. "And the order didn't come."

DILEMMA

When the President's top advisers learnt what happened, they were flabbergasted. Mr Pompeo was described as incredulous, Mr Bolton as aggravated.

The advisers were stunned all over again the next morning when Mr Trump took to Twitter to reveal that he had been "cocked and loaded" for a strike and then called it off. There was no need to publicly disclose how close they had come to acting, they thought; by doing so, the President risked making it look like he had blinked.

Some said Mr Trump has only himself to blame. "Trump is in a box of his own making," said Mr Philip Gordon, who was a Middle East adviser to Mr Obama.

"He has put in place policies - 'maximum pressure' on Iran - guaranteed to provoke an aggressive Iranian response, but he's not prepared to respond aggressively in turn, and the Iranians know it," said Mr Gordon. "So now he has to either back down or go down that slippery military slope, a terrible dilemma he should have considered before he went down this road in the first place," he added.

NYTIMES