From President John F. Kennedy’s time in office through President Barack Obama’s, new treaties were negotiated that first limited atmospheric testing, then the numbers of weapons that could be deployed. The result was an 85 percent decline in the number of nuclear weapons in the world.

But the most recent arms reduction treaty, called New Start, expires in early 2021. Mr. Trump has not expressed interest in renewing it. And there is evidence that even before its scheduled sunset, expensive new cycles of technological action and reaction are beginning to take shape.