This year’s presidential campaign has featured relatively little detailed discussion of nuclear policy. That changed—a little—in Monday night’s debate.

Republican U.S. presidential nominee Donald Trump and Democratic U.S. presidential nominee Hillary Clinton speak during their first presidential debate, September 26, 2016. REUTERS/Adrees Latif

The final segment of the Hillary Clinton-Donald Trump debate, vaguely titled “Securing America,” focused in significant part on the nuclear threat. The discussion--directed largely by the candidates themselves--was substantive. And because both candidates have political reasons to continue to address the existential threat of nuclear weapons, the debate could serve as a much-needed starting point for elaborating on the topic during the remaining two presidential debates.

Monday’s nuclear discussion grew out of a question about cyber security that gradually morphed into arguments about the best ways to combat Islamic State. Clinton suggested that to protect the American people from terrorism, the United States needed an “intelligence surge” that required cooperation with U.S. allies. “We’re working with NATO, the longest (lived) military alliance in the history of the world, to really turn our attention to terrorism. We’re working with our friends in the Middle East, many of which, as you know, are Muslim-majority nations,” Clinton said. “Donald has consistently insulted Muslims abroad, Muslims at home, when we need to be cooperating with Muslim nations and the American Muslim community.”

“We’ve been working with them for many years, and we have the greatest mess anyone’s ever seen,” Trump responded. “You look at the Middle East. It’s a total mess, under your direction to a large extent... You started the Iran deal, that’s another beauty. Where you have a country that was ready to fall, they were doing so badly. They were choking on the sanctions, and now they’re going to be actually, probably a major power at some point pretty soon, the way they’re going.”

The exchange led to a curious detour in the debate, during which Trump tried to take credit for inspiring NATO to fight international terrorism and repeatedly contended he did not advocate the 2003 invasion of Iraq, despite widely reported evidence to the contrary. Given that evidence, asked moderator Lester Holt, why was Trump’s judgment about the invasion any different than Clinton’s? “I have much better judgment that she does. There’s no question about that. I also have a much better temperament than she has,” Trump replied. The audience guffawed softly.

Odd though it was, the interlude led back to a discussion of important nuclear policy issues. After pouncing on Trump’s assertions about NATO and terrorism (pointing out the alliance’s response to 9/11, which included sending troops to Afghanistan), Clinton touted her role in the early stages of a 2015 agreement to limit the Iranian nuclear program. She then criticized what she characterized as Trump’s tendency toward bellicose rhetoric.

“He has said repeatedly that he didn’t care if other nations got nuclear weapons -- Japan, South Korea, even Saudi Arabia,” Clinton said. “It has been the policy of the United States, Democrats and Republicans, to do everything we could to reduce the proliferation of nuclear weapons. He even said, ‘Well you know if there were nuclear war in East Asia, well you know that’s fine...” “Wrong,” Trump interjected.

“...have a good time folks,” continued Clinton. “And in fact his cavalier attitude about nuclear weapons is so deeply troubling. That is the number one threat we face in the world, and it becomes particularly threatening if terrorists ever get their hands on any nuclear material.”

Trump vigorously disagreed with Clinton’s portrayal of his views on nuclear proliferation, saying that he only advocated that U.S. allies pay their fair share of the cost of defending them. He also echoed her nuclear concerns: “I agree with her on one thing: The single greatest problem the world has is nuclear armament, nuclear weapons. Not global warming like you think, and your president thinks. Nuclear is the single greatest threat.”

The debate’s last comments on nuclear weapons came in regard to U.S. policy on when they can be used, a complex issue that neither candidate took on squarely. Current U.S. policy allows the first use of nuclear weapons under certain dire circumstances; President Obama has reportedly considered the possibility of adopting a policy under which Washington would pledge never to use nuclear weapons first (although recent reports suggest he is not leaning toward making such a policy change). When Holt asked Trump about his views on a no-first-use policy, Trump said Russia had been expanding its nuclear forces with “a much newer capability” than the United States has.

“I would like everybody to end it, just get rid of it. But I would certainly not do first strike,” Trump said ambiguously. “I think that once the nuclear alternative happens, it’s over. At the same time we have to be prepared. I can’t take anything off the table.”

Clinton avoided the first-use question entirely, saying the United States should honor its mutual defense treaties and again defending the Iran nuclear deal.

Both candidates were right on Monday night: Nuclear weapons are the most urgent threat the citizens of the United States and the world face. The United States and Russia have thousands of nuclear warheads, many of them on missiles kept at high states of alert. Each of these warheads is sufficient to obliterate a city. The use of even a small fraction of these weapons could end civilization as we know it by causing worldwide cooling dubbed “nuclear winter.”

How many nuclear weapons should the United States have? Should they be kept on hair-trigger alert? Should the country spend an estimated $1 trillion over 30 years to modernize the bombers, land-based missiles, submarine-launched missiles, and warheads of the nuclear triad? (More questions from top nuclear experts can be found here:here.)

The discussion probably isn’t over yet. The Clinton campaign would like to paint Trump as someone who cannot be trusted to command nuclear weapons. Trump wants to show he is aware of the danger of nuclear war and up to the task of preventing it. If the moderators of the next two presidential debates are interested in sparking a useful discussion on the greatest threat to the continued existence of humanity, they will likely find candidates willing to engage and an audience -- in the United States and around the world -- eager to continue existing.