North Korean Foreign Minister Ri Yong-ho on Monday threatened to shoot down US warplanes, claiming “the right to shoot down United States strategic bombers even when they are not inside the airspace border of our country.” It’s one of the most serious threats the North has leveled to date — and it has experts extremely worried.

“This is how war by miscalculation starts,” Vipin Narang, a professor at MIT who studies nuclear weapons, tweeted after seeing Ri’s comments. “My anxiety level is up sharply today.”

The reason for his increased anxiety is clear: The US frequently flies warplanes over the Korean Peninsula. Ri’s comments appear to be a direct response to such a flight conducted on Saturday, in which US B-1B bombers flew along the North Korean coast while remaining in international airspace. It was the farthest north of the demilitarized zone between North and South Korea that any US fighter jet or bomber has flown in the 21st century, according to the Pentagon.

The idea behind these flights is to deter a war, not start one. They’re designed to show the North Korean government that the US is willing to use force if it does something provocative, and thus deter the North from trying anything.

But the recent tensions between President Donald Trump and North Korean leader Kim Jong Un seems to be making the North more aggressive, not less. North Korea’s foreign minister’s comments suggest the country may no longer tolerate this kind of defensive signaling from the US, and may begin treating these flights as aggressive acts of war even if they don’t cross into North Korean airspace.

An actual exchange of fire between North Korea and the US — two nuclear-armed powers — is still unlikely, but the fact that it is much more plausible today than it was yesterday is disturbing.

“I would take Ri's words seriously. This is where we are now,” Jenny Town, the assistant director of Johns Hopkins’s US-Korea Institute, tells me. “I'm not sure how we walk back from this without some serious diplomatic efforts.”

This is the fruit of badgering a nuclear-armed rogue state

The root of the current standoff, experts explain, is something called the “stability-instability paradox.”

Here’s how it works: Nuclear weapons can deter war, as we observed during the Cold War. The US and the Soviet Union worked hard to avoid outright conflict because no one believed they could win a nuclear war. In that sense, nuclear weapons enhance stability.

But the sense of security that nuclear weapons grant — because who in their right mind would attack a nuclear power? — can also encourage lower-level bad behavior. In 2010, for example, a North Korea submarine sank a South Korean destroyer, the ROKS Cheonan, without things escalating to war. The North gambled that the South wouldn’t risk being hit by Northern nukes (and its conventional arsenal) over one destroyer, and so wouldn’t respond with all-out war. It was right.

This paradox — where nuclear weapons deter full-scale war but at the same time encourage lower-level provocations — is why Kim thinks he can get away with threatening, and perhaps even firing on, US bombers.

“We're in first strike instability territory”

Jeffrey Lewis, an expert on North Korea at the Middlebury Institute for International Studies, compares this dynamic to what happened when car manufacturers first began putting seat belts in cars: “There is some research about seat belts — early on, it seems, drivers with seat belts drove more aggressively,” Lewis says. “Nuclear weapons, for some leaders, do the same thing.”

North Korea hasn’t fired on any US warplanes since becoming a nuclear power in 2006, despite the US conducting many defensive flights like the one on Saturday. The reason it’s flexing its muscles now, experts say, is that Trump’s threats — like his tweet on Sunday warning that North Korea “won't be around much longer!” if it keeps threatening the US — makes the North wary that the B-1B flights might be a prelude to an actual bombing run.

“DPRK really hates the B-1B flights,” Narang tweeted. “They're clearly making the regime nervous about surprise attack.”

Now the Trump administration has two choices: stop doing these flights and look like you’re bowing to the North’s threats, or keep doing them and risk an actual exchange of fire. If the administration chooses the latter, then what happens if Pyongyang isn’t bluffing and actually fires on a US warplane? Does Trump back down, or does he respond with a strike of his own?

Lewis calls this scenario “the nightmare I’ve been warning about,” in which a war no one wants becomes plausible. Other experts agree.

“The B-1 threat definitely escalates,” Mira Rapp-Hooper, a North Korea scholar at Yale University Law School, tells me. “We're in first strike instability territory.”

Again, this does not mean a US-North Korea war will break out tomorrow. It does not even mean a war is likely. But it does suggest that North Korea’s threat this morning is not its typical bluster, but may be a qualitative shift in the nature of US-DPRK tensions — one that moves us from a vague war of words to having a very specific scenario in which a hot war could start. And that should trouble everyone.