WASHINGTON — Donald J. Trump often says he never wants to signal to the nation’s adversaries what he would do as commander in chief — an embrace of the concept of “strategic ambiguity” that is as old as warfare.

But on the critical question of whether the United States should ever be the first to use nuclear weapons, he appeared somewhere between contradictory and confused during his debate with Hillary Clinton on Monday.

On a related issue that Mr. Trump raised himself — the modernization by the Russians of their nuclear arsenal at a time when the United States is still debating plans and looking for the money — he had the gist of it right. The implication of his observation was that the United States had to match what the Russians were doing, creating the specter of a new arms race; Mrs. Clinton hinted at that, too, but in the arena of cyberweapons.

Questions about Mr. Trump’s philosophy on the use of nuclear weapons have shadowed him throughout the campaign, in part because Mrs. Clinton has repeatedly raised them to sow doubts that he has the temperament for a job in which he would have sole authority over the world’s most powerful arsenal.