If U.S. intelligence discovers that North Korea has a nuclear tipped, long-range missile getting ready to fire into the South Pacific with the goal of detonating it—to prove to the world it is a nuclear power—there is only one thing President Trump should do: destroy it before it ever goes into the air.

And that could mean war with North Korea—a war that I have waged many times in computer simulations and in tabletop exercises and it frightens me to depths of my soul. That could mean millions of people dead. But the alternatives, even with the stakes so high, are too frightening to imagine.

America and its allies would simply have no choice but to respond to what is the most dangerous of threats the world has faced since the Cuban Missile Crisis.

Consider for a moment if we let Kim Jong Un go ahead with such a missile launch.

For one, consider if Kim gives the order, and it heads into the skies above Northeast Asia. However, all does not go as planned, and the missile’s guidance system has a problem or the engines fail. The missile could then head towards the ground and crash into a populated area in South Korea or Japan. While the warhead on the top of the missile, holding a potential atomic or hydrogen bomb, might not detonate on impact, the potential for radioactive contamination and fallout over a wide area is very possible. And if the worst happens, an actual nuclear detonation, the path to war seems as if it would be mapped out in atomic lettering.

The Trump administration simply cannot allow North Korea to begin lobbing nuclear weapons across sovereign nations and use the Pacific Ocean as its own personal atomic testing grounds.

Then there is the danger if North Korea’s nuclear test is successful, making a long voyage across the Pacific. Imagine a North Korean ICBM heading towards the South Pacific, passing over Japan, maybe even Guam and detonating in an unpopulated, isolated part of the planet. While the potential loss of life would be low, the long-term impacts could be felt across the globe. Radioactive fallout would be carried by the winds and oceans all over both sides of the Pacific, potentially causing cancers and illness not detected for years. Pacific island populations could be greatly impacted, some of which won’t have the resources to handle such a crisis. There is a reason why nations do not test nuclear weapons in the atmosphere anymore—there could be no greater poison to our planet.

Then there is something far greater to fear—that other states will see this as a greenlight to conduct their own atmospheric tests at some point in the future, or simply to build nuclear weapons, as they know Washington is powerless to stop them.

Or North Korea, if not stopped from doing such a test, could do it again…and again. And considering that Pyongyang has already fired missile and rockets over Japan, can we really believe that would stop at one?

Now, to be very fair, I realize that taking out a North Korean missile before it launches is very likely an act of war. Also, being someone who is against unilateral military action against Kim Jong Un to destroy his nuclear weapons program before it can definitively strike the United States, this might even come across as a flipflop of sorts—I get it. However it may seem, the Trump administration simply cannot allow North Korea to begin lobbing nuclear weapons across sovereign nations and use the Pacific Ocean as its own personal atomic testing grounds. While I don’t ever like the term redline with all of its historical baggage, this is one Team Trump cannot allow to be crossed.

So how would North Korea respond to such an attack? As someone who has studied Pyongyang for over a decade, I want to be one-hundred-percent honest: I don’t know. And that terrifies me.

Pyongyang could decide to respond by shelling downtown Seoul with hundreds of artillery shells and missiles. Kim could decide to sink another South Korean naval vessel, just like he did a few years ago. Or he could test another ICBM, but this time lob it a few hundred miles off the West Coast of the United States. He could even decide—however remote—that now is the time to invade South Korea and achieve his family’s long-lost dream of reunifying the Korean peninsula under communism. Maybe he decides now is the time for general nuclear war—even more remote—but not out of the woods of considerations. Heck, he could even do nothing—but I doubt it.

The above quick survey of America’s options in dealing with this latest potential provocation with North Korea and how Kim could respond proves only one thing: there are no good options. But to allow Kim Jong Un to openly test nuclear weapons in the Pacific Ocean, flying them over sovereign nations with all that can go wrong, is even more insane then taking them out. The potential pitfalls that could occur only leaves one option, if it comes to it.

We can only hope the pariah of Pyongyang was just bluffing, as it would be time for America to call.