Maybe McMaster wasn’t articulating administration policy; other Trump advisers have hinted that deterrence can work with North Korea. But consider what it means if McMaster was. If North Korea can’t be deterred, then North Korea must be denied the world’s deadliest weapons—particularly, from the U.S. perspective, nukes that can target America—by any means necessary. Since we’re hurtling toward a time when North Korea will have these capabilities (if they don’t have them already), and since international efforts to coax and coerce North Korea off this course have so far failed, McMaster’s logic leads to some dark places.

As Colin Kahl, the former national-security adviser to Vice President Joe Biden, put it shortly after McMaster made his comments, if North Korea is undeterrable, and a North Korea capable of threatening the United States with nuclear weapons is thus intolerable, then war to prevent North Korea from posing that threat seems inevitable.

What Exactly Is Deterrence?

The concept of deterrence can be whittled down to a simple message, according to Mira Rapp-Hooper, an expert on Asian security issues at Yale University: “If you do X thing that I don’t like, I will do Y thing that will be even worse for you.” For deterrence to work, whoever is considering X must take seriously the threat of Y and be “rational” enough to fear it. If one country doesn’t fire nukes at another because its leaders believe the second country will respond with a devastating nuclear counterattack, then deterrence is operating as it should. Nuclear deterrence is a subset of general deterrence, which encompasses various methods of discouraging hostile actions.

“In the annals of international relations, there are [few] theories that have a better track record” than nuclear deterrence, Fred Kaplan writes in Slate. That track record, in brief: No conflict involving nuclear weapons in the 70-plus years since the U.S. bombings of Hiroshima and Nagasaki.

The subtext of McMaster’s remarks to ABC is that the Kim regime is so hostile and brutal that it isn’t rational enough to fear punishment. But McMaster hasn’t offered much evidence for that assessment. The United States and its allies, after all, have successfully deterred other hostile, brutal governments from using nuclear weapons, including Joseph Stalin’s in the Soviet Union and Mao Zedong’s in China. And if North Korea’s leaders are really irrational—if they don’t respond to incentives and disincentives as other leaders would—why is the Trump administration using economic sanctions and other forms of pressure to try and change their calculus on developing nuclear weapons? (The National Security Council did not respond to a request for comment from McMaster.)

Given that missile-defense systems are unreliable, the United States has for decades been practically “defenseless” against the nuclear-tipped long-range missiles possessed by Russia and China, Robert Gallucci, a former Clinton administration official who negotiated with the North Koreans in the 1990s, noted earlier this year. But it has relied on deterrence purchased with the strongest military in the world. The Trump administration, Gallucci wrote, should “disclose what exactly is new about the North Korean threat that makes deterrence suddenly unreliable. Certainly it is not the quality or quantity of North Korea’s nuclear weapons. At the height of the Cold War, the number of Soviet weapons … reached 30,000 or so. The North Koreans have less than 20.”