Securing Postwar Ukraine, Even With Trump’s Pledge to Help, Is Complex

President Trump has pleased Ukrainian and European leaders by promising American involvement in providing security guarantees for Ukraine if a peace settlement with Russia ever comes together.

Mark Rutte, the NATO secretary general, pronounced himself “excited” over Mr. Trump’s public commitment on Monday at a summit at the White House to some sort of security guarantee, a pledge that the Europeans have been eagerly seeking. He called it “a breakthrough.”

But exactly what those guarantees would involve remains ambiguous. Officials promised more clarity in the weeks to come as defense ministry planners come to grips with the considerable complications of turning a broad promise into realistic options.

Mr. Trump said that European countries would be the “first line of defense” in providing security guarantees for Ukraine, but Washington will “help them out, we’ll be involved.” He added later: “European nations are going to take a lot of the burden. We’re going to help them and we’re going to make it very secure,” he said.

He did not explain how.

Some involved, like Prime Minister Giorgia Meloni of Italy and the president of the European Commission, Ursula von der Leyen, spoke of an “Article 5-like” guarantee outside of NATO itself, though based on the commitment in the alliance’s charter that an attack on one member is considered an attack on all of them.

But it is hard to imagine that NATO itself would not be quickly implicated if any member state of the alliance with troops stationed in Ukraine gets into a shooting war with Russia.