LAS VEGAS—Hillary Clinton had one of her best moments in any of the three presidential debates on Wednesday when she quickly and terrifyingly explained how easy it is for any president—even one named Donald Trump—to start a nuclear war.

Here is her explanation of why a Trump with nuclear buttons should scare all of us:

This is a person who has been very cavalier, even casual about the use of nuclear weapons. He has advocated more countries getting them: Japan, Korea, even Saudi Arabia. He said well “if we have them why don’t we use them,” which I think is terrifying.

And here is her explanation of how easy it would be for President Trump to fire nuclear weapons. This answer is devastating:

But here’s the deal: The bottom line on nuclear weapons is that when the president gives the order, it must be followed. There is about four minutes between the order being given and the people responsible for launching nuclear weapons to do so. And that is why 10 people who have had that awesome responsibility have come out and in an unprecedented way said they would not trust Donald Trump with the nuclear codes or to have his finger on the nuclear button.

The fear among Trump opponents—or expectation among some of his supporters—that he will get us into a nuclear conflict is a real one. A SurveyMonkey poll last month found that nearly half of American voters thought a President Trump would use a nuclear weapon, including 22 percent of Trump supporters.

Outside of the press parking lot at the Thomas and Mack Center, the site of Wednesday’s debate, one protester came out to explicitly express this concern.

Clinton supporter Allan LeTourneau was holding a sign that read: “Vote Donald Trump for Nuclear War.”

LeTourneau told me he genuinely fears that America would engage in a nuclear conflict under a President Trump. “It scares the hell out of me,” he said. “For the world’s sake.”

LeTourneau basically expressed the same concern as Clinton:

He’s indicated that he likes nuclear weapons, he’s indicated that other countries should have them and he asked why we haven’t used them. It’s scary and the guy is out for revenge anytime somebody steps on his toes, he goes after them any way he can.

Here’s what those former launch officers that Hillary cited said:

The pressures the system places on that one person are staggering and require enormous composure, judgment, restraint and diplomatic skill.

Donald Trump does not have these leadership qualities. On the contrary, he has shown himself time and again to be easily baited and quick to lash out, dismissive of expert consultation and ill-informed of even basic military and international affairs—including, most especially, nuclear weapons.

Read more Slate coverage of the 2016 campaign.