Retired Navy Adm. James Stavridis spent 37 years in the military, including four years as the supreme allied commander of NATO. Hillary Clinton vetted him as a possible running mate. President-elect Donald Trump considered naming him secretary of state. He is a serious man, and about as far from an armchair pundit as it’s possible to be.

And that’s precisely what makes his assessment of the escalating standoff with North Korea so jarring. Stavridis believes there’s at least a 10 percent chance of a nuclear war between the US and North Korea, and a 20 to 30 percent chance of a conventional, but still bloody, conflict.

“I think we are closer to a significant exchange of ordnance than we have been since the end of the Cold War on the Korean peninsula,” he said during a panel I moderated Tuesday at the University of Pennsylvania’s Perry World House.

His estimate of the potential death toll from even a nonnuclear war with North Korea is just as striking. North Korea has at least 11,000 artillery pieces trained on Seoul, South Korea’s capital of 25 million people, and would be certain to use them during any conflict. The US would be just as certain to mount a sustained bombing campaign to destroy those artillery pieces as quickly as possible.

The result? “It’s hard for me to see less than 500,000 to 1 million people, and I think that’s a conservative estimate,” he said.

Remember: That’s assuming North Korea doesn’t use its arsenal of nuclear weapons, which can already hit Seoul and much of Japan.

Speaking at the same event, Michèle Flournoy, formerly the No. 3 official at the Pentagon in the Obama administration, said Trump’s harsh rhetoric toward Pyongyang — which has included deriding North Korean leader Kim Jong Un as “Little Rocket Man” — created the real risk of an accidental war between the two countries.

“My worry is that all of this heated rhetoric has really charged the environment so that it’s much more likely now that one side or the other will misread what was intended as a show of commitment or a show of force,” she said. “It could be the basis of a miscalculation that actually starts a war that wasn’t intended at that moment.”

Flournoy currently runs a centrist think tank called the Center for a New American Security (full disclosure: I wrote a book while working as a writer in residence there from 2012 to 2013), and Stavridis is the dean of the Fletcher School of Diplomacy at Tufts University. Neither is gloomy by nature, or prone to alarmism.

And that’s precisely why I left the stage feeling so gloomy, and so alarmed.

Here’s why the odds of war with North Korea are rising

Both Stavridis and Flournoy see Kim as a fundamentally rational leader whose overriding goals are to ensure the survival of his regime and his personal control over North Korea. Nuclear weapons, in Flournoy’s words, are “the ace that he could play if there was a conflict to say, ‘Stop, you’re not going to take me out without risking nuclear war.’”

Stavridis stressed on the panel that the odds were still against an open military conflict with North Korea, let alone nuclear war. But he also made clear that both were definitely possible — and that the odds were rising.

“I think there’s a 10 percent chance the wheels really come off and we have a full-on war on the Korean Peninsula, which would include nuclear use,” he said. “That’s well over double what it was three months ago.”

The retired admiral — who helped oversee the wars in Iraq and Afghanistan during his years running NATO — said the chances of a lower-level conflict were even higher, at 20 to 30 percent. He laid out a scenario for how one could break out.

“The Koreans try or actually shoot down a US aircraft. We respond, maybe taking out a bunch of Korean ships in [their] harbor,” he said. “But somehow we manage to contain it and don’t escalate to that full-blown war.”

Flournoy said that preventing that sort of conflict from erupting would require both countries to take tangible steps back from the brink. Trump would need to tone down his rhetoric and send a high-level envoy to China to make clear to Beijing that the North Korean crisis was rapidly approaching a point of no return. Kim would have to agree to some sort of freeze on his testing and development of new nuclear weapons and more advanced long-range missiles.

Stavridis responded as you might have expected, given his cheerful talk of nuclear war with North Korea.

“I think the chances of that are roughly the same of Elvis Presley walking in here right now,” he said.