If David Cameron had a replay of last week’s parliamentary vote, he would have waited for Barack Obama to announce first. Whether the US president’s weekend decision to seek Congressional approval first occurred to him before or after Mr Cameron’s stunning reversal is now moot.

Even though he now risks being accused of prevarication, Mr Obama is right to seek authorisation. He is also right to couch the request for military action in narrow and limited terms. But he needs to do a far better job of explaining his broader strategy on Syria. Failure to spell it out could jeopardise the clear majorities he will need in both chambers of Congress. Defeat in either chamber would deal a grievous blow to his presidency.

The case for US-led international action is strong and becoming stronger. By killing at least 1,000 civilians with sarin gas last month, the Assad regime has traduced the most basic humanitarian norms. It could do so again. Failure to register the world’s disapproval – and its willingness to respond – would embolden the Assads and their supporters. In contrast to Mr Cameron’s cobbled-together approach, the Obama administration has made a coherent, forceful case for limited, punitive strikes. On the draft resolution’s language alone, Mr Obama deserves a Yes vote from Capitol Hill as soon as practical – preferably before the end of next week.