On Tuesday, President Donald Trump signaled that his historic summit with North Korean leader Kim Jong Un might not happen. “There’s a very substantial chance it won’t work out,” Trump told reporters. “I don’t want to waste a lot of time, and I’m sure he doesn’t want to waste a lot of time.”

It’s not just Trump — North Korean leaders are increasingly raising doubts about whether the meeting will happen. “If the US is trying to drive us into a corner to force our unilateral nuclear abandonment, we will no longer be interested in such dialogue and cannot but reconsider our proceeding to the DPRK-US summit,” said Kim Kye Gwan, vice foreign minister, on May 15, using the official acronym for North Korea.

So it seems the summit is not 100 percent guaranteed to take place. That brings up vital questions: What happens if the summit falls through — or if it takes place but the two sides leave the negotiating table without resolving the decades-old nuclear standoff between Washington and Pyongyang?

Those are distinct possibilities given that the two sides have laid out positions that seem difficult, if not impossible, to resolve. Secretary of State Mike Pompeo, for instance, told the House Foreign Affairs Committee on Wednesday that he had told Kim the Trump administration wants the “complete, verifiable, and irreversible denuclearization” of North Korea, meaning Pyongyang fully dismantles its nuclear program.

That’s likely a no-go for Kim, according to a recently released Pentagon report about North Korea, because “Pyongyang portrays nuclear weapons as its most effective way to deter the threat from the United States.” In other words, Kim has no incentive to give up his nukes because he feels they’re the only thing keeping Washington from invading and trying to overthrow his government.

With both sides so far apart on a deal, there’s reason to suspect Trump and Kim might not meet. The problem is even experts aren’t sure what happens then.

“At this point, it is easier for me to predict the exact weather on June 12 than it is to predict what will happen with the summit,” Alexandra Bell, a former State Department adviser for nuclear policy, told me.

From a possible summit to a possible war

In an interview, MIT nuclear expert Vipin Narang ranked four potential scenarios, from best to worst, about what could happen between now and June 12.

First, both sides delay the summit because they realize they’re too far apart on the denuclearization issue. That would give Pompeo and lower-level staffers more time to bridge the yawning gaps. And if they come to some sort of understanding, then the leaders can meet to finalize a deal. This, by the way, is how diplomacy usually works.

Second, Washington and Pyongyang delay the summit but make no significant progress toward an agreement because of how far apart they are on ending Kim’s nuclear program. The status quo holds, but relations between the two countries remain better than they were in past decades because they’ve now held direct talks.

Third, “the temperature rises just as quickly as it falls and we go back to 2017,” Narang said.

That means North Korea could return to testing missiles, and the US may increase sanctions. Recall that just last year Trump promised to rain down “fire and fury” on North Korea if it kept threatening the US and its allies. Kim, in response, vowed to “surely and definitely tame the mentally deranged US dotard with fire” for openly talking about destroying North Korea.

And fourth, those words become reality as the United States and North Korea do the unfathomable: They go to war. The Trump administration pursues “denuclearization by force,” Narang said, “using the failed summit as evidence that diplomacy is futile.” But of course, he noted, “that would mean attacking a nuclear weapons power.”

Narang told me he thinks the second scenario — a delayed summit with no diplomatic progress — is the most likely one. Perhaps more importantly, he believes a full-blown war is the least likely, in part because Pyongyang has nuclear weapons, making an invasion very risky because the North could use them on America or regional allies South Korea and Japan.

But let’s be clear: If the summit falls through, the risks of war — no matter how low the odds — go up.

Why war is by far the worst option

Pompeo echoed Trump when he told the House Foreign Affairs Committee on Wednesday that “a bad deal is not an option. The American people are counting on us to get this right. If the right deal is not on the table, we will respectfully walk away.”

If Trump’s team thinks only a bad deal is on offer, or Trump and Kim can’t even agree to meet, then the administration may go back to escalating tensions with Pyongyang. The two sides are already threatening each other again. On Monday, Vice President Mike Pence said North Korea could end up like Libya — ruthlessly bombed by Western airpower and with its leader forcibly deposed — “if Kim Jong Un doesn’t make a deal.”

That comment shows the specter of war between North Korea and the United States hasn’t gone away. My colleague Yochi Dreazen reported on what a horrific outcome that conflict would be. Here’s just a taste:

The nonpartisan Congressional Research Service estimates that Kim could hammer the South Korean capital with an astonishing 10,000 rockets per minute — and that such a barrage could kill more than 300,000 South Koreans in the opening days of the conflict. That’s all without using a single nuclear, chemical, or biological weapon... Kim would try to level the playing field by using his vast arsenal of chemical weapons, which is believed to be the biggest and most technologically advanced in the world. (Kim is estimated to have between 2,500 and 5,000 metric tons of deadly nerve agents like sarin, which can cause paralysis and, ultimately, death.)

As Dreazen ultimately finds, “A new war on the Korean Peninsula wouldn’t be as bad as you think. It would be much, much worse.” And he notes that you could expect that millions — plural — would die.

War, then, is by far the worst alternate option should there be no June 12 summit. Most experts hope Trump and Kim will stick with a diplomatic approach for as long as possible.

“Leaders in the region and around the world should be leaning on the United States and North Korea to pursue substantive, pragmatic dialogue,” says Bell, now a nuclear expert at the Center for Arms Control and Non-Proliferation. “It’s the only way to solve this crisis.”

The question is whether Trump and Kim continue to believe that — and whether they’re willing to actually sit down together to see if there’s a deal to be had.