As McMaster tells it, traditional deterrence (if you strike us, we’ll strike you), which helped dissuade the Soviet Union from firing nuclear weapons at the United States during the Cold War, may not work with a government as brutal as Kim Jong Un’s. If the world’s most despicable regime isn’t prevented from acquiring the world’s most destructive arms, what’s keeping other nations from racing to build their own nuclear arsenals? North Korea, which has exported missiles and nuclear-related materials to countries such as Iran and Syria, could sell nuclear weapons to America’s enemies, he warns. And a nuclear North Korea could blackmail U.S. leaders by, for example, threatening to incinerate Los Angeles unless America withdraws support for its ally South Korea, exposing the South to invasion by the North.

In brief, North Korea’s development of a long-range nuclear capability “would be the most destabilizing development ... in the post-World War II period,” McMaster says. (Bill Rapp, a Harvard lecturer and retired major general who studied and worked alongside McMaster for more than three decades at West Point and in Iraq and Afghanistan, told me that McMaster views his role in the administration as overseeing a “disciplined process” for presidential decision-making, not as being an “independent voice for policy.” McMaster’s office did not respond to interview requests for this article.)

According to Nagl and many other international-affairs scholars, however, the principles of deterrence can work just as well against a nuclear-armed North Korea as they have against the far more formidable nuclear powers of Russia and China. “The only thing I can imagine is that somehow [the Trump administration has] a different picture of Kim Jong Un’s regime, of the pressures it’s under, of its desire to go out with a bang,” Nagl told me. “But I can see no evidence to support that. ... I see North Korea pursuing a defensive mechanism to preserve [its] regime.”

So why does McMaster seem to feel differently? There are clues in two texts he’s referenced repeatedly—The Unquiet Frontier, a 2016 book by the scholars Jakub Grygiel and Wess Mitchell, and “The Rhyme of History,” a 2013 essay by the historian Margaret MacMillan. Both paint a dire picture of the world—and America’s role in it.

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The Unquiet Frontier, which McMaster reviewed favorably and whose authors now serve in Trump’s State Department, asserts that U.S. rivals are “probing” for weakness at the edges of American power. “The Rhyme of History” argues that today’s technological and geopolitical upheaval resembles that which preceded World War I. What binds them together is a world-historical assessment that the post-World War II international system is in flux, that immutable realities of geography and great-power competition are reasserting themselves after an exceptional era of harmony in the 1990s, and that the United States would be wise to leap into the breach to defend its interests and allies against states bent on challenging the status quo. As McMaster noted in a recent speech in Washington, D.C., “Geopolitics are back ... with a vengeance, after this holiday from history we took in the so-called post-Cold War period.”