Why is H.R. McMaster so alarmed by North Korea? Why does Donald Trump’s national-security adviser insist—more vigorously than any administration official except the president himself—that Kim Jong Un must be denied the capability to place a nuclear warhead on a missile that can reach the United States, even if this requires initiating a military conflict with the North that could devolve into a cataclysmic war?

While Secretary of State Rex Tillerson and Secretary of Defense James Mattis are focused on diplomatic efforts to curb North Korea’s nuclear program, McMaster “is arguing more vocally, publicly and privately, that military options need to be considered,” The Wall Street Journal noted on Tuesday. The Trump administration, it reported, is debating whether to give Kim a “bloody nose” by conducting limited strikes against North Korean targets in retaliation for further nuclear or missile tests.

A legendary tank commander during the Gulf War and one of the on-the-ground architects of U.S. counterinsurgency strategy in the second Iraq War, McMaster burnished a reputation as one of the Army’s leading thinkers about the future of war. He wrote a dissertation on the mistakes of military and political leaders during the Vietnam War and frequently warns that today’s world resembles the world on the precipice of World War I. And, to one former collaborator, something seems very off about McMaster’s talk of potential war with North Korea. “I find it absolutely inexplicable—not in keeping with the man I know, with his writing, with his thinking, with the sense of responsibility he feels for preserving peace and security and innocent life,” said John Nagl, a retired lieutenant colonel who worked with McMaster on the U.S. Army’s Counterinsurgency Field Manual during the Iraq War.

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.”

In this reckoning of what the 21st century has in store, deterrence is less straightforward than it once was, which helps clarify McMaster’s concerns about protecting Americans from a nuclear-armed North Korea. U.S. allies and adversaries around the world are watching to see whether the United States passes or fails the test of wills on the Korean peninsula, which helps account for McMaster’s worries about sparking a nuclear-arms race and losing America’s alliance with South Korea and foothold in East Asia. And the very international system that the United States remade in its image 70 years ago is under severe strain, which helps explain why McMaster has set the stakes for the North Korean nuclear crisis so high.

“It’s almost impossible to overstate the threat from a nuclear-armed North Korea,” McMaster declared at the Virginia Military Institute just days before Trump won the White House, well before he had reason to believe he would be serving as national-security adviser to a president pledging to meet the North Korean threat with “fire and fury” if necessary.

Grygiel and Mitchell describe a world in which rivals are testing U.S. commitments in historical “hingepoints” from Central Europe to East Asia—from Russia’s invasion of Ukraine to China’s island-building in the South China Sea. Grygiel and Mitchell don’t focus on North Korea. But in a 2011 article for The American Interest, as an example of what these often-subtle trials look like, they cited China’s resistance to cracking down on its North Korean ally: “a kind of ‘probing by proxy’ not unlike Russia’s use of its Balkan allies to probe the balance of power in the region in the first decade of the 20th century,” prior to World War I.

The United States doesn’t need to respond aggressively to every probe, Grygiel and Mitchell argue, but it must “drive up the costs of revisionism” where its interests and values are most threatened. They call for reassuring “frontline” allies with U.S. weapons and an enhanced U.S. military presence. Grygiel and Mitchell refer to this as deterrence “by denial” (making it more difficult for the revisionist to achieve its military objectives in the potential conflict zone), which they distinguish from traditional deterrence “by punishment” (threatening to retaliate against the revisionist if it attacks the United States or U.S. allies). The latter approach, according to Grygiel and Mitchell, has been degraded as America’s commitment to its allies has come under question and as revisionist states have come to favor limited, ambiguous acts of aggression that don’t rise to the level of an outright attack and trigger punishment. Today’s revisionist powers are “salami-slicers” when it comes to U.S. deterrence, they write, and the United States must inflict “visible” and “proportionate” pain on the aggressor when the salami gets sliced.

The consequences of failing to sufficiently counter these probes are spelled out in stark terms. “Not since the 1930s has the world witnessed the emergence of multiple large, predatory states determined to revise the global order to their advantage—if necessary by force,” Grygiel and Mitchell write. “At a minimum, the United States in coming years could face the pressure of managing several deteriorating regional security spirals; at a maximum, it could be confronted with a Great Power war against one, and possibly two or even three, nuclear-armed peer competitors.”

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MacMillan focuses on a different period, the lead-up to World War I, to illustrate a similar point about the dangers of the present moment. Now, as then, the “guarantor of international stability”—Britain in the early 20th century, the U.S. in the early 21st—seems less willing and able to carry out its duties. Now, as then, hostilities between “lesser powers” threaten to draw their great-power patrons into direct conflict. Now, as then, people have misplaced faith in the notion that “large-scale, all-out war” is a thing of the past. Now, as then, arms buildups are mistaken as developments that will promote peace rather than the reverse. MacMillan wonders whether the deterrence that succeeded when the United States and Soviet Union had a near-monopoly on the world’s nuclear weapons can still function in a world in which nine countries possess nuclear arsenals, including North Korea.

“It’s more complicated perhaps than it was in 1914 because that was pretty much all about rivalry between powers whereas now we do have that, but we also have these very confusing wars with shifting sides,” MacMillan told me, likening modern conflicts in Afghanistan, Iraq, and Syria to the Thirty Years’ War. “We’ve got a combination of pre-1914 and 1618 to 1648.”

McMaster has often echoed these themes. In a speech previewing the Trump administration’s new National Security Strategy, he compared contemporary “threats to liberty and freedom” from terrorist groups, the “revisionist powers” of China and Russia, and the “rogue regimes” of Iran and North Korea to the threats that the United States and its allies previously faced from “fascism, imperialism, and communist totalitarianism.” Britain, he noted, was “the guarantor of European security in a way that is somewhat analogous to the United States’ role today in coping with revisionist powers on the Eurasian mainland who are challenging our influence at the far reaches of our power—along what Jakub Grygiel and Wess Mitchell have called ‘the unquiet frontier.’”

“I do believe, geopolitically, that this period of time is analogous to 1914,” McMaster observed in his 2016 address at the Virginia Military Institute, before urging the audience to read MacMillan and Grygiel and Mitchell. Traditional deterrence doesn’t work as well as it used to since China, Iran, North Korea, and Russia “often act below the threshold that would elicit a concerted response from [the United States] and our allies,” he added. “These hostile actors do not operate in isolation from one another. They watch and assess American actions and responses across the globe. They calibrate their actions.”

McMaster’s spirited defense of the U.S.-led international system seems at odds with Trump’s challenges to that system. A man who has credited the post-World War II order with averting great-power conflict for seven decades works just paces away in the West Wing from a boss who has reportedly concluded that the postwar order is “not working at all.” But each advocates the rugged pursuit of U.S. interests in a rough-and-tumble world—which, judging by their rhetoric on North Korea, apparently means seriously considering the use of force as a last resort.

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What’s most striking about McMaster’s position on North Korea is that he has repeatedly dangled the possibility of preventive military action against the North despite his acute awareness of how unpredictable and potentially all-consuming such a conflict could be. Even if the United States initially carries out a limited “bloody nose” strike—designed not to eliminate the North Korean nuclear program, but to signal that the United States is serious about rolling back the program—the mission might spiral into an all-out war against the Kim regime that experts estimate could involve weapons of mass destruction and leave thousands, even millions, dead.

McMaster has argued in the past that no matter how much precise, high-tech weaponry and remote naval and air power a country has, there are certain conflicts where low-cost, lightning-fast victories are an illusion and where there is no substitute for land forces. “Hard fighting in complex terrain will be needed to prevail” in a conflict on the Korean peninsula, he wrote in a 2005 article for Joint Force Quarterly.

“In the aftermath of Desert Storm, before Iraq really went to hell, H.R. was one of the people … saying that the world cannot be solved with airstrikes,” said Nagl. “You can achieve short-term temporal effects from a distance,” Rapp added. “But you may not achieve the long-term effects that you were hoping to achieve.”

McMaster has also characterized war as inherently human and uncertain—“interaction between opposing wills that, when combined with violence, chance, and emotion, make the future course of events impossible to predict,” as he put it at the Virginia Military Institute. “War is very contingent, it’s very non-linear, it takes weird turns that you weren’t expecting because you’re up against a thinking adversary,” Rapp explained.

The North Korean nuclear program presents the Trump administration with “a fearful dilemma,” MacMillan observed: “Can [the U.S. government] take the risk that this crazy little power will get the capacity to deliver nuclear weapons to the United States, which it seems to be close to getting? But on the other hand how does it stop it? A land war?”

“The trouble with North Korea is we’re dealing in the dark,” she continued. “At least in 1914 [the countries involved] knew something about each other. All the major belligerents had ambassadors in each other’s capitals before the war broke out.” The hazards lie in “the decisions people make assuming the other side is going to behave in a certain way. The Germans and the Austro-Hungarians kept on saying to themselves, ‘Russia will back down. It’s backed down before. They’re just bluffing.’ And this time the Russians weren’t bluffing.”

Which is why it’s puzzling that McMaster nevertheless appears willing to take that risk when it comes to Kim Jong Un. “A man who understands the dynamics that led to the First World War as well as H.R. does should be even more cautious about the idea of preventive war with the North Korean regime,” Nagl said. “Because you don’t know what’s going to happen when you put that kind of energy into the system. … We give North Korea a ‘bloody nose.’ They respond with a conventional artillery strike on [the South Korean capital of] Seoul. We go nuclear. China mobilizes [to prevent the Kim regime’s fall]. … There is every prospect of a ‘bloody nose’ for North Korea ending in global war between China and the United States.”

McMaster’s position would make more sense if the Trump administration’s bellicose rhetoric is a big bluff—an attempt to pressure China into isolating North Korea and the North into making concessions—which is certainly possible, Nagl noted. But he added that, in his experience, McMaster isn’t a bluffer: “What H.R. says you can take to the bank.”