Such a “reckless” attack by the regime of Bashar al-Assad and its Russian and Iranian allies, he tweeted, would be a “grave humanitarian mistake” that could kill “hundreds of thousands of people”.
It would make the world and the US “very, very angry”, Mr Trump added later.
This came after months of signalling that he was fed up with US engagement in Syria: in April, he began talking about bringing American troops home, and last week, he cancelled $230m (£178m) earmarked for repairing war damage in the country.
So was the Idlib intervention just an impulsive tweet, or did it mark a change in strategy?
On the one hand, Mr Trump’s comments sounded similar to the way he has responded to chemical weapons attacks that have twice led to limited US air strikes on Syrian regime targets.
In those cases, he reacted emotionally to the disturbing images of children asphyxiated by poison gas. He was also eager to better his predecessor, Barack Obama, who has been criticised for drawing a red line on Syria’s use of chemical weapons and then…