President Donald J. Trump was right to strike at the regime of Syrian President Bashar al-Assad for using a weapon of mass destruction, the nerve agent sarin, against its own people. Mr. Trump may not want to be “president of the world” but when a tyrant blatantly violates a basic norm of international conduct — in this case, the ban on using chemical or biological weapons in armed conflict, put in place after World War I — the world looks to America to act. Mr. Trump did, and for that he should be commended.

The real test for Mr. Trump is what comes next. He has shown a total lack of interest in working to end Syria’s civil war. Now, the administration has leverage it should test with the Assad regime and Russia to restrain Syria’s air force, stop any use of chemical or biological weapons, implement an effective cease-fire in Syria’s civil war and even move toward a negotiated transition of power — goals that eluded the Obama administration.

At the same time, it must prevent or mitigate the possible unintended consequences of using force, including complicating the military campaign against the Islamic State. All this will require something in which the administration has shown little interest: smart diplomacy.

That smart diplomacy starts with Russia. The administration reportedly previewed the strike with Moscow. Cynics might conclude the fix is in: The United States quietly warns the Russians, they give Mr. Assad a heads-up and tell him not to react, and everyone calls it a day. More likely, the administration wanted to make sure Moscow knew exactly what we were doing so that Moscow would not overreact or leave its forces in harm’s way.