President Trump speaks during a Cabinet meeting at the White House on Monday. (Nicholas Kamm/AFP/Getty Images)

HAVING DECLARED that Syria will pay a “big price” for its latest use of chemical weapons, President Trump will deal another blow to U.S. global leadership if he does not follow through. But a few cruise missiles won’t change anything in Syria. What’s really needed is a concerted strategy for protecting the vital American interests wrapped up in the multi-sided Syrian war — something Mr. Trump, despite the urging of many of his advisers, has failed to develop.

At the least, Mr. Trump should learn a lesson from this latest Syrian war crime. He declined to respond to seven previous, smaller chemical attacks this year. Then he loudly announced he intended to pull out U.S. forces and “let the other people take care of” Syria. He should not have been surprised that the ever-opportunistic regime of Bashar al-Assad responded by dumping toxic chemicals on the Damascus suburb of Douma. More than 500 people, most of them women and children, were treated for symptoms, and at least 48 died. Mr. Trump, who criticized President Barack Obama for allowing red lines in Syria to be crossed with impunity and for telegraphing military plans in advance, ought to recognize that the Assad regime and its Russian and Iranian allies are as happy to take advantage of his fecklessness as they were of Mr. Obama’s.

The chemical attack appeared to have achieved its tactical aim: Opposition fighters who had been holding out in Douma reportedly agreed to be evacuated with their families to the northern province of Idlib, which is still rebel-controlled. Eventually the regime will launch a scorched-earth campaign against that province, too, with the help of Russian bombers and Iranian-led militias. In that sense, a U.S. military response to the Douma attack could serve a minimal purpose if it deters the regime from employing chemicals in future offensives. That would require punishment comparable to that inflicted by Israel in recent raids, which took out a big part of the Syrian air force and, on Monday, targeted Iranian forces on a Syrian base.

But the reality Mr. Trump has not yet faced is that as long as the dictator he called “Animal Assad” remains in place, Syria’s wars will continue, breeding Islamist terrorists and propelling refugees toward Europe. Mr. Trump does have an advantage that Mr. Obama lacked: Thanks to the capture by U.S. and allied forces of a large part of eastern Syria, the United States has the capacity to stabilize at least part of the country and has leverage in demanding an acceptable outcome to the war. The costs could be relatively modest — Mr. Trump recently balked at $200 million in stabilization funds for Syria, a tiny fraction of the tens of billions spent on reconstruction in Iraq and Afghanistan.

If Mr. Trump really intends to abandon Syria, he should be prepared not to flinch at chemical attacks and other war crimes. There’s little point in one-off punitive raids if there are to be no U.S. military or diplomatic initiatives behind them. Wars cannot be fought by impulse. They require something Mr. Trump has yet to embrace: a plan.