Donald Trump points to supporters during the opening session of the Western Conservative Summit in Denver.  (David Zalubowski/AP)

Donald Trump's most infamous explanation of who guides his foreign policy thinking came during an interview with NBC's Chuck Todd last summer.

"Who do you talk to for military advice right now?" Todd asked.

"Well, I watch the shows," Trump replied, referring to appearances from former members of the military on Sunday political talk shows like Todd's. Later in the campaign, he rolled out a short list of actual advisers — a list that included a man who graduated from college in 2009 and listed Model United Nations as one of his credentials.

It's not clear whether one of those advisers was the person who whispered to "Morning Joe" host Joe Scarborough about Trump's questions during a briefing earlier this year. During a segment on the show Wednesday, Scarborough relayed a story from an unnamed individual about the Republican nominee.

"I'll be very careful here. Several months ago," Scarborough said, "a foreign policy expert on an international level went to advise Donald Trump, and three times he asked about the use of nuclear weapons. Three times [Trump] asked — at one point, 'If we have them, why can't we use them?' "

This is the often-melodramatic Scarborough's retelling of a story he heard from someone else, so grains of salt should be sprinkled. But Trump has toyed with the idea of using nuclear weapons in the past. As CBS's Sopan Deb notes, Trump asked MSNBC's Chris Matthews in March, "Why are we making them?" if we never plan on using them. That was the same interview in which he said he wouldn't rule out using nukes in the Middle East — or, for that matter, in Europe. Our fact-checking team documented several other times Trump refused to rule out the use of nuclear weapons.

We'll note that the list of military advisers was revealed during an interview with The Washington Post's editorial board in March. Publisher Fred Ryan asked Trump twice whether he'd use a nuclear weapon against the Islamic State. Trump's reply the second time was, "I’ll tell you one thing, this is a very good-looking group of people here. Could I just go around so I know who the hell I’m talking to?"

If Trump is actually wondering why we don't use nuclear weapons, we got an answer from Kennette Benedict, former executive director and publisher of the Bulletin of the Atomic Scientists.

"We actually do use them: as a deterrent," Benedict said when we spoke with her by phone Wednesday. "That's been U.S. policy for 50 years at least, maybe longer, just to have them and for people to know that if they attack us or attack any of our NATO allies, we're prepared to use them — if it comes to that, as a last resort."

If Trump meant to wonder why we don't actually explode them, Benedict offered two scenarios. If we were to attack a nation like Russia with nuclear weapons — that is, another nation with nukes of their own — we'd precipitate a nuclear war, with devastating effects for our economy, infrastructure and the residents of major cities. This is the doomsday that those of us who grew up in the Cold War remember looming.

Even the use of a tactical nuke against the Islamic State is deeply problematic, she said. "A tactical nuclear weapon is about anywhere from 10 to 50 times the explosive power of the Hiroshima bomb," she said. "Any place that would be assaulted by such a weapon would be completely devastated" — devastation that would include the deaths of thousands of people, including, almost necessarily, civilians, and that would leave deadly radiation and fallout in the region for years following.

"Our standing in the world would be radically altered if we were the first to use nuclear weapons," she said, "and if we were to use them in this way, that's completely disproportionate to the situation."

On "Morning Joe," Scarborough — brow duly furrowed — followed up his story about Trump's nuclear wondering with a question to former NSA head Michael Hayden. How long, he asked, does it take to actually launch a nuclear weapon?

"The system is designed for speed and decisiveness," Hayden replied. "It's not designed to debate the decision."

Meaning that the idea of the president having a direct finger on the button is not that far from reality.

Update: It's worth reading this thread from someone who used to work at the facility that controls nuclear strikes.