Exactly a year ago, the Americans responded to a chemical attack in Syria by bombing an airbase from which the planes thought to have delivered the gas had flown. The action was surprising, given Donald Trump’s predilection for staying out of the Middle East conflict, other than to confront the threat to US security posed by the Islamic State group. The White House drew comparisons between the strike and the inaction of the previous US administration, which had drawn red lines against the use of chemical weapons in Syria but then abandoned them.

Now that there is seemingly irrefutable evidence of an even more horrific use of gas on rebels in the suburb of Douma near Damascus, and President Trump...