Vipin Narang is an associate professor of political science at the Massachusetts Institute of Technology, specializing in nuclear proliferation and strategy, and a member of MIT’s security studies program

North Korea's nuclear weapons program is advancing quickly. This year, it has tested a suite of short- and medium-range ballistic missiles that can hit neighbors and American bases in East Asia, submarine-launched ballistic missiles, two intercontinental-range ballistic missiles and a purported thermonuclear weapon capable of flattening a city. Soon Kim Jong Un will be able to deliver it to our shores, if he cannot do so already.

This, we are told, is an unfortunate but not an existential problem. Although it will reshape geopolitics, there is no real threat of nuclear warfare, because Kim has no death wish. Secretary of State Rex Tillerson says Americans should sleep well at night, and analysts argue that it would be tantamount to national suicide for Kim to use nuclear weapons against the United States. If his purpose is to ensure his survival (how better to understand his quest for nukes?), why would he risk it by starting a conflict with Washington he can't win? Surely it won't come to war, let alone nuclear war.

Yes, Kim is brutally rational. And that is precisely why he may have to use nuclear weapons, but not in a first strike against American cities. Kim's nuclear arsenal exists to stop his enemies' quest for regime change. If North Korea and the United States wind up shooting at each other, it might make sense for Kim to use nuclear weapons first in a way that increases his chances of survival. The basic idea is to use one set of nuclear devices to stave off the conventional invasion, and hold in reserve longer range, more powerful devices that threaten the enemy's cities to deter nuclear annihilation. It's a doctrine called "asymmetric escalation," employed by states that are conventionally weak. France articulated it during the Cold War to deter the more powerful Soviet Union, and Pakistan does the same today against a more powerful India.

[Diplomacy really can work against North Korea. Here’s why.]

The strategy turns on Kim's main calculation that the United States will say it's not worth losing a major American city to get rid of him. This would allow him to avoid the fate of Iraq's Saddam Hussein and Libya's Moammar Gaddafi, who did not have nuclear weapons. Deterrence worked uneasily during the Cold War — albeit with close calls and some hair-raising moments — but it worked. Many of the same principles about mutual destruction still obtain today between major powers.

Yet the equation for North Korea, which cannot ensure mutual destruction, is slightly different. Faced with the prospect of a U.S.-led invasion, Pyongyang's conventional inferiority requires it to degrade the United States' ability to sustain the attack against it. This means it essentially has no option but to use nuclear weapons first against targets such as Andersen Air Force Base in Guam, which stations American bombers, and a variety of allied bases in Japan and South Korea. North Korea has to use nuclear weapons there because it does not have enough conventional warheads to damage the bases meaningfully; a conventional response would not slow or stop a U.S. onslaught. It is for these bases that North Korea has tested the medium-range missiles, reportedly developed a compact nuclear fission warhead and honed guidance for the missiles that would carry it.

Wouldn't such an attack mean the retaliatory annihilation of North Korea? Not necessarily. This is why the intercontinental ballistic missile (ICBM) and the H-bomb are so important. Kim's survival theory is that North Korea could threaten to destroy an American city with a thermonuclear-tipped ICBM if the United States continued an invasion or retaliated with nuclear weapons. Anytime its cities can be held at risk, the United States' deterrence equation changes, as it did during the Cold War. Are we willing to risk losing millions of civilians in our homeland? Possibly not. And it's unlikely that we could reliably destroy all of Kim's ICBMs on the ground or intercept the warheads in the air, particularly as he builds more. So the prospect of losing San Francisco thanks to our nuclear retaliation may cause us to pause conventional operations and elicit a cease-fire, thereby preserving Kim's regime and rule. Kim may surmise that if he doesn't use nuclear weapons first, he is certain to lose; if he does, he may have a fighting chance of surviving.

This scenario to stave off an invasion with a limited nuclear attack on a U.S. military target is not irrational, although it is clearly risky and terrifyingly tragic. One wrinkle is that North Korea's arsenal is currently small and vulnerable, and U.S. military strategy, reiterated by Defense Secretary Jim Mattis, is to try to find and destroy all of Kim's nuclear systems in the event of a war. That gives Kim an incentive to go first, go early and go massively if he is not confident about surviving a U.S. attempt at disarming him. If Kim thinks we are coming after him or his forces, he cannot afford to be wrong, and he cannot afford to launch second.

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States with small arsenals that are put under counterforce pressure have itchy trigger fingers. It is what is known as the use-it-or-lose-it dilemma. Prior to World War I, European powers believed they all had to mobilize military forces first or risk massive conventional defeat. The calculation for North Korea is the same today, except with nuclear weapons.

This current risk is amplified by our saber-rattling. How do we assure Kim that the B-1B sorties from Guam that are meant as "shows of strength" are not a prelude to a counterforce surprise attack? We are in a particularly dangerous phase right now, and not because Kim is unpredictable. The more rational he is, the itchier his trigger finger could be.

At the broader political level, Kim has another aim with his nuclear weapons: to break our alliances. The Soviet Union's acquisition of ICBM technology caused panic among our allies. France developed its own nuclear weapons, because Charles de Gaulle was convinced we would not trade Pittsburgh for Paris. Today, the concern among our allies is that with our homeland at risk, we might not trade San Francisco for Seoul, or Toledo for Tokyo. These anxieties are amplified when President Trump accuses South Korea and China of "appeasement" after North Korea's thermonuclear test. Pyongyang probably read that tweet with glee, thinking that its political strategy is already working. With a nuclear security umbrella like the one we maintain in East Asia, it's always harder to reassure allies than it is to deter the adversary. Right now, we are being outplayed by Kim on both counts.

[I live on Guam. Here’s how we’re coping with the nuclear standoff.]

Dispensing with the notion that Kim is crazy or irrational is important for two reasons. First, it clarifies the military and political strategies he might envision with nuclear weapons. Second, it suggests that he responds to both domestic and international incentives. It means deterrence — which was always coupled with reassurance and diplomacy — can work with North Korea, just as it did with the Soviet Union and China. But deterrence works both ways: We can no longer threaten to attack North Korea without risking a nuclear exchange.

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