But no one in his or her right mind would invade North Korea. While irrationalities associated with the North Korean regime—and perhaps, in the regime’s analysis, with President Trump—could raise the question of whether Pyongyang recognizes this, the U.S. has signaled repeatedly over decades that it does not want a confrontation, let alone a war. America has dramatically reduced its ground troops on the peninsula; it has withdrawn tactical nuclear weapons from South Korea; and it has engaged in repeated negotiation efforts. And the North Korean quest for missiles that can hit the U.S. far predates the Trump presidency. Furthermore, everyone in leadership on both sides also knows the background. The last U.S. invasion, in October 1950, provoked Chinese military intervention and the biggest battlefield defeat the U.S. Army ever suffered. Even without nuclear systems able to hit the U.S., Kim’s nuclear and huge conventional arsenal suffices to deter any invasion.

But if his goal is to conquer the South, holding the U.S. as nuclear hostage gives him a strategic advantage that threatening Seoul with conventional artillery would not. Moreover, conquest is less phantasmagorical than it appears. Unification on its terms has long been the official policy of Pyongyang (and of South Korea, but it does little to advance it), and it is not just theory—Kim’s grandfather, Kim Il Sung, sacrificed perhaps a million Koreans for that end in the1950 invasion. The Washington that sneers at such a scenario once doubted that North Vietnam, with a political system similar to North Korea’s, would sacrifice even more to unify with its south.

The “book” on how to deal with threats that may not be susceptible to classic nuclear deterrence was written during the Cold War, and McMaster is applying it, not ignoring it. In the classic model, both sides have sufficient nuclear systems to survive another side’s first strike and retaliate with a second strike—so launching a nuclear attack is suicide. This, for example, is the decade-long U.S.-China nuclear posture, but it’s insufficient to reliably deter war when real or perceived conventional superiority and aggressive intent are also in play. The USSR after World War II initially sought nuclear weapons as deterrence, but also maintained massive conventional forces in Eastern Europe to keep open the option of conquering Western Europe. The U.S. had pulled its ground troops almost entirely out after 1945, and President Dwight Eisenhower, leery of breaking the bank on a huge peacetime American ground force in distant central Europe, decided on a strategy of massive retaliation—the U.S. would respond not with tanks but nuclear weapons to an invasion.

But by the 1960s the Soviets could hold the American homeland under nuclear threat. This raised the question: Would Americans sacrifice their own cities to save European? Equally important, would its allies believe they would? French President Charles De Gaulle famously did not. Kennedy saw this threat and increased U.S. ground forces, but could not hope to match Soviet forces. So the U.S. remained focused on an “automatic” escalatory ladder beginning with tactical nuclear weapons deployed at the front, many under “dual key” arrangements with NATO allies (meaning that the use of tactical nuclear weapons, deployed to alliance members like Germany or Turkey, had to be approved both by the U.S. and the ally in question, further complicating Soviet calculations). In any conflict, such weapons in the middle of a chaotic battlefield, some partially controlled by countries facing extinction, inherently contained a huge risk of a nuclear spiral even if the U.S. didn’t want a global nuclear war. But the planners sought precisely to maintain such a credible “balance of terror” despite the vulnerability of the U.S. homeland.