Why did Assad use nerve gas in Idlib? It’s impossible to know. Maybe it was a signal to an increasingly aggressive Israel that he still had chemical weapons, or maybe it was a warning to Russia that he wasn’t a pawn to be traded in a grand bargain with Trump. But most likely, it was a reaction to the free hand he was seemingly given when Secretary of State Rex Tillerson said a few days earlier during a visit to Turkey that Assad’s future “will be decided by the Syrian people” — meaning that the United States no longer demanded his departure.

Trump has now taken a decisive step that Obama resisted, but he still faces a dilemma of how to bring political change to a Syria shattered by six years of civil war. The irony is that Trump faces the same bad military options for pressing the attack in western Syria beyond this initial strike that Obama did. U.S. military commanders have focused on the battle against the Islamic State in eastern Syria. The Pentagon has a plan for a relatively rapid conquest of Raqqa, the de facto capital of the caliphate of the Islamic State, with a force led by Syrian Kurdish fighters. U.S. troops will augment this assault with heavy artillery, Apache helicopter gunships and forward-deployed advisers. It’s a pragmatic plan for ending the Islamic State terrorist threat quickly, even at the risk of offending Turkey and leaving Assad and the political muddle in Damascus. This plan had all the pieces except a final White House stamp of approval.

Then came those pictures of the Syrian children. With Thursday night’s action, Trump reached one of those unforeseen tipping points on which decisions of war and peace so often rest: the sinking of the Lusitania in 1915, the “Zimmermann telegram” of 1917, Pearl Harbor in 1941, the Gulf of Tonkin attack in 1964, the Iraqi WMD delusion in 2003. What all these triggers for war have in common is that people didn’t see them coming.