The top nuclear commander in the U.S. said Saturday that he would reject an “illegal” nuclear attack order from President Donald Trump, and would instead steer the commander in chief to other “options.”

“If you execute an unlawful order, you will go to jail,” Air Force Gen. John Hyten told an audience at the Halifax International Security Forum in Nova Scotia. “You could go to jail for the rest of your life.”

Hyten, the commander of the U.S. Strategic Command who oversees America’s nuclear arsenal, spoke at a forum titled “Nukes: The Fire and the Fury,” as recorded in a video on the event’s Facebook page. He didn’t define what exactly would constitute an illegal launch order. But Hyten said he has been trained for the past 36 years in the law of armed conflict, and mentioned the consideration of such elements as proportional response and unnecessary suffering that would be caused by such a conflagration.

In the event Trump suggests an illegal strike, Hyten described a scenario in which he would present Trump with legal choices.

“I provide advice to the president. He will tell me what to do,” Hyten said. “And if it’s illegal, guess what’s going to happen? I’m going to say, ‘Mr. President, that’s illegal.’ And guess what he’s going to do? He’s going to say, ‘What would be legal?’ And we’ll come up with options... a mix of capabilities to respond to whatever the situation is, and that’s the way it works. It’s not that complicated.”

“I think some people think we’re stupid,” he said. “We’re not stupid people. We think about these things a lot. When you have this responsibility, how do you not think about it? What people forget is that this is a military mission and military function.”

In congressional testimony earlier this month, retired Gen. Robert Kehler, who served as the head of Strategic Command from 2011 to 2013, also said that the military is only obligated to follow legal orders.

Hyten’s comments, first reported by CBS News, come in the wake of taunting threats against North Korea by the president. In August, Trump told reporters that if North Korea continued to threaten the U.S., its actions would be met “with fire and fury the likes of which this world has never seen before.” Many interpreted this to mean a nuclear strike.

Trump also threatened in a United Nations speech to “totally destroy” the country of 26 million people if its leader, Kim Jong Un, threatened the U.S.

“This is a very serious subject and the world is a very dangerous place right now,” Hyten said Saturday.

The general said his command must provide “strategic deterrence.” If that fails, “we’ll have a decisive response,” he said, and must be “combat-ready to do that all the time.”

Hyten’s job, he said, is to “create the conditions for diplomacy to work by being ready all the time, and we are ready every minute of every day.”

The forum Saturday included a video that showed scenes of nuclear devastation beneath the song “We’ll Meet Again” ― the same music used at the end of Stanley Kubrick’s 1964 antiwar film “Dr. Strangelove,” which famously ends with a nuclear holocaust. Hyten said he has seen the movie nearly half a dozen times.

Hyten referred to a 2016 speech by then-President Barack Obama calling for a nuclear-free world. While he seemed moved by the speech, Hyten said, “I don’t believe we’ll ever get to a nuclear-free world.”