The first sign of a change came on Wednesday afternoon, when Donald Trump appeared in the White House rose garden. He was giving a news conference. The previous day a rocket had fallen on the rebel-held Syrian town of Khan Sheikhun.

At first this appeared to be another airstrike by the Syrian regime of Bashar al-Assad on an area in Idlib province long controlled by opposition forces. It was, seemingly, a routine act of barbarism. An airstrike was unremarkable in Syria’s grinding six-year-long civil war.

And yet this one was different. The missile allegedly contained the deadly nerve agent sarin. Within minutes, the northern part of the town had been engulfed with poisonous gas, a choking yellow fog. At least 70 people were killed: men, women, children.

The first rescuers came on a scene that Trump described as “horrific”. Children lay on the ground, foaming at the mouths, their lips going blue, dropping in and out of consciousness.

Inside houses, teams found infants dead in their beds. Near them were the bodies of victims that had collapsed as they tried to flee.

“Wherever you looked there were dead human beings,”

said Abu al-Baraa, who lives nearby .Previously Trump’s administration had indicated that its priorities in Syria were fighting Isis and Islamist terrorism. There was no mention of removing Assad from power: a key, long-unfulfilled demand from the US’s European allies. And then there was Russia, Assad’s chief backer, and a power that from 2015 had tilted the war in the regime’s favour.

A few days earlier Sean Spicer, the White House press secretary, said it was necessary to accept “political reality” in Syria. Up to this point Trump had been a longstanding sceptic of US military action. Back in 2013 he tweeted that the president, Barack Obama, should refrain from intervening and “stay away and fix a broken US”. The “rebels”, he added, were “just as bad as the Syrian regime”.Now, though, Trump was talking about his new “flexibility”. “I will tell you that attack on children yesterday had a big impact on me – big impact,” Trump told reporters in the rose garden. “My attitude toward Syria and Assad has changed very much … You’re now talking about a whole different level.”

This was the first alleged attack by the regime using sarin since 2013, when the nerve agent was dumped on an opposition-controlled area of Damascus. More than 1,000 people perished. This latest attack – after a deal brokered by Russia, in which Assad agreed to give up his chemical weapons stockpile – “crossed a lot of lines for me”, Trump said.

He went on: “When you kill innocent children, innocent babies, babies, little babies, with a chemical gas that is so lethal – people were shocked to hear what gas it was. That crosses many, many lines, beyond a red line, many, many lines.”

The president, in office for just 77 days, declined to say whether he would now contemplate the previously unthinkable: a US strike against Syrian military targets. This would go beyond anything done by the Obama administration, which came close to an attack in 2013 but then blinked. Trump had criticised Obama for vacillation and weakness. Would he act differently?

Within 24 hours Trump’s flexibility had translated into direct action. At the Pentagon senior military officials began drafting policy options. They included both extensive and less far-reaching strikes against Syrian regime targets. There was diplomatic movement too: the secretary of state, Rex Tillerson, signalled that the White House was considering a decisive response.

On Wednesday evening Trump’s core national security team held discussions on Syria which went on until the early morning. There were conversations involving Trump’s national security adviser, HR McMaster – a more conventional figure than his shortlived predecessor, Mike Flynn – plus the defence secretary, Jim Mattis. Tillerson was looped in.