Mr. Trump touched on most of those — except the uncle — on Monday night, but seemed to come down on both sides of the nuclear-first-use debate.

“I would certainly not do first strike,” Mr. Trump first declared, appearing to be advocating a change in American policy that dates to the Truman administration.

But he did not stick with it. “I think that once the nuclear alternative happens, it’s over,” Mr. Trump said. “At the same time, we have to be prepared. I can’t take anything off the table.”

That put him pretty much where Mr. Obama ended up after seven and a half years. The president vowed to reduce the role of nuclear weapons in American strategic planning, and he did.

It was not the first time Mr. Trump has been asked about nuclear first-use, or the first time he offered an obfuscated answer.