In some of the more memorable remarks of his tenure as President Donald Trump’s national security adviser, H.R. McMaster has ruled out the idea of keeping Kim Jong Un in check through nuclear deterrence. In August, he rhetorically asked, “the classical deterrence theory, how does that apply to a regime like the regime in North Korea?”

If there were any doubt about the answer, McMaster declared in October that the president is “not going to accept this regime threatening the United States with nuclear weapons. There are those who would say, well, why not accept and deter. Well, accept and deter is unacceptable.”

These remarks lend added force to T. Negeen Pegahi’s keenly observed essay on the decline of deterrence in U.S. national security strategy — a phenomenon that, if anything, she understates. Furthermore, the underlying aversion to the concept is not new. As Joshua Rovner has discussed, America’s historic pattern has been to embrace deterrence only grudgingly and belatedly. This, too, may be an understatement.

Who actually subscribes to the strategy of deterrence, as it is understood by academic theorists? Among defense policymakers, few do, and few ever have. Judging by the number of references to “deterrence” and “strategic stability” in policy documents, one might assume otherwise. But words are supple. Whatever purposes might be imagined for nuclear weapons, it is hard to justify such extraordinary power to destroy in anything but defensive-sounding terms. In truth, what is routinely called the “nuclear deterrent” is not necessarily meant for deterrence alone.

This resort to euphemism is hardly unique to deterrence. In the United States, we are accustomed to fighting our wars abroad, but we still call the big building with five sides “the Department of Defense.” And it’s exactly there, in the Pentagon, that the reasoning of the nuclear strategists has never really been accepted.

So what do we mean by “classical deterrence theory”? Definitions vary, but there are two essential ideas: one that arose in the aftermath of the bombings of Hiroshima and Nagasaki, and a second, complementary idea that took form over a decade later as the Soviet Union tested its first intercontinental ballistic missiles. Viewed against the background of established military thinking, each idea was revolutionary in its own right.

The first idea might be called security through overwhelming retaliation. In early 1946, with the first Soviet nuclear test still years away, the strategist Bernard Brodie offered perhaps the earliest description of the concept. By virtue of their extraordinary destructive power and the impossibility of comprehensively preventing their delivery, he wrote, nuclear weapons were, on balance, “a powerful inhibition to aggression.” The key was the ability to retaliate. Holding annihilating striking power in reserve, and keeping it secure from surprise attacks, transformed it into a new kind of defense.

“The first and most vital step in any American security program for the age of atomic bombs,” Brodie wrote, “is to take measures to guarantee to ourselves in case of attack the possibility of retaliation in kind.” With a “good chance” of retaliation assured, any enemy capable of envisioning the likely outcome would refrain from attacking. So the thinking went, and so it goes.

This strategy was, to say the least, counterintuitive. First, despite relying upon awesome powers of destruction, deterrence was defensive in spirit. Second, it warded off attack not through actual destruction, but through the enemy’s fear of destruction. Winston Churchill described these aspects of nuclear deterrence as a “process of sublime irony” through which “safety will be the sturdy child of terror, and survival the twin brother of annihilation.” In a similar spirit, the NATO alliance adopted as its informal mascot not a lion or an eagle, but the hedgehog, a “peaceful creature” that “bristles when attacked.”

But as Soviet power grew, so did doubts about the sturdiness of the so-called “balance of terror.” Sputnik, among other developments, put a sharper point on the first basic requirement of deterrence: preserving the ability to retaliate. As Soviet capabilities grew, defense intellectuals like Albert Wohlstetter saw the need to upgrade America’s strategic nuclear forces, which appeared to be increasingly vulnerable to surprise attack.

Other strategic thinkers, like Thomas Schelling, went further still, distilling the second core concept of deterrence: stability. A roughly symmetrical nuclear standoff, they realized, meant that both sides had to concern themselves with possible surprise attacks — no matter who might be tempted to strike first! If either side’s arsenal posed a credible threat to the other’s, the opponent would feel pressure to preempt, lest it be utterly wiped out. Furthermore, an awareness of these circumstances would also pressure the first side to make the first move — an inherently dangerous situation for both sides.

It was insufficient merely to assure one’s own power to retaliate. Each side’s confidence in its own ability to retaliate, and its confidence in the other side’s ability to retaliate, were equally important. With the emergence of two superpower nuclear arsenals, “stability” became the second crucial consideration for preventing a nuclear war.

But are these the concerns that have animated American strategy? The evidence suggests that they have never been fully embraced. Instead, many defense policymakers have been drawn to the idea of nuclear superiority, which has an intuitive appeal even when it finds little justification in terms of deterrence. Other, perhaps more mainstream planners have favored the idea of “damage limitation.” This means, in essence, preparing to fight a nuclear war — just in case — to wipe out as much of the enemy’s nuclear forces as possible, leaving America as intact as possible. This pursuit comes at the expense of stability, which involves giving both sides as much confidence as possible that they will not come under attack.

Most Americans would probably agree that reducing the harm of a nuclear war is a good idea, but not if that also means boosting the chances that such a war will occur in the first place. Nevertheless, the close-knit group that has shaped the development of the nuclear arsenal has consistently favored “counterforce” capabilities, optimized for fighting nuclear wars. This policy operates on the assumption that deterrence cannot be preserved — a view that risks becoming a self-fulfilling prophecy.

Counterforce proponents nevertheless see fit to dress their approach in the language of deterrence. Indeed, perhaps no one was more accustomed to speaking of “deterrence” euphemistically than Curtis LeMay, the Air Force general who led the Strategic Air Command from 1948 to 1957, its formative years. He later served as chief of the Air Staff. While LeMay’s peak influence overlapped with the “golden age of strategy” that produced the classic works on deterrence, his own approach could hardly have been more different.

In 1956, LeMay publicly disputed the views of his superior, the secretary of the Air Force, who had remarked at a conference that nuclear superiority was not a requirement for ensuring the ability to retaliate. To this, LeMay said, “You have to have more combat potential [than the enemy] if you are going to be a deterrent force.”

The evidence suggests that despite using the language of deterrence, which implicitly points toward assuring retaliation — threatening to strike back after an attack — LeMay was personally committed to nuclear preemption, or attacking the enemy first in order to wipe them out. In 1957, the general allegedly rejected experts’ concerns about the vulnerability of the bomber force, insisting if that he saw the Soviets preparing to attack, he would hit them first.

As late as 1960, LeMay lobbied against the Navy’s program for creating a highly secure undersea retaliatory force. “Forces and measures which might in some way help to deter the start of war are not necessarily those that could win a war if the deterrent fails in its purpose,” he said.

Winning a war by striking first — as LeMay and the other Joint Chiefs of Staff urged President Kennedy to do with conventional air power during the Cuban Missile Crisis — is still controversial. But the belief that nuclear forces must be designed with the failure of deterrence in mind continues to dominate the thinking of the nuclear establishment.

Early in the Cold War, strategic nuclear forces and war plans grew much larger than the simple idea of ensuring retaliation seemed to warrant. In later decades, even as both superpowers’ arsenals have declined in size, they have become increasingly sophisticated, featuring highly accurate and reliable missiles capable of launching promptly and reaching their targets in minutes. Recent scholarship suggests that the United States, during the entirety of the Cold War, never abandoned efforts to undermine the survivability of Soviet nuclear forces — in other words, to prepare for a first strike.

Even the Obama administration, which had considered declaring deterrence of nuclear attacks to be the “sole purpose” of nuclear weapons, insisted upon “maintain[ing] significant counterforce capabilities against potential adversaries.” The enduring fascination with missile defenses to defend the homeland from nuclear attack conveys the same attitude: a deeply ingrained dissatisfaction with nuclear deterrence.

So much for nuclear deterrence. What about conventional deterrence? The undeniable superiority of American military power after the Cold War helped to revive interest in this idea, which is based less on retaliation than on “denying” the enemy any opportunities for successful aggression. But just as the Gulf War taught the United States lessons about the persistence of nuclear proliferation risks, it helped to crystallize an approach to defense planning based on overmatching adversaries and defeating them in detail. This preference has relegated conventional deterrence to an afterthought.

Post-Cold War joint doctrine has been modeled implicitly — sometimes explicitly — on the one-sided triumph of Operation Desert Storm. It closely informs the “keystone document” of U.S. military doctrine, JP 3-0, in particular Chapter V, “Joint Operations Across the Conflict Spectrum.” This chapter provides a phased model of military activities, from peacetime “shaping” of the security environment (Phase 0) to “deterrence” (Phase I), through “seizing the initiative,” achieving “dominance,” “stabilization” of occupied areas, and finally “enabling civil authorities” prior to withdrawal. Within the Pentagon, expressing oneself in terms of these phases is simply inescapable.

In this model, “deterrence” is not a general strategy for achieving national security or meeting alliance commitments; it is merely a set of activities that occur as part of the preparations for war. In a deeper sense, the phased model in JP 3-0 presumes a major failure of conventional deterrence, one that necessitates a military campaign to reverse an enemy’s fait accompli. It treats “deterrence” merely as making one’s war preparations sufficiently visible to the enemy to encourage, hopefully, a painless surrender. As former Chairman of the Joint Chiefs of Staff Colin Powell observed in an oral history of the Gulf War, massing and employing a “decisive force” to impose one’s will upon the enemy is almost always the best path to ensuring the least costly victory possible.

The central idea of joint doctrine, then, is not deterrence; it is war. The preferred approach is nothing less than the mobilization of overwhelming military power, with every intention of setting it into motion. But it is difficult to understand how to apply this strategy to a nuclear-armed opponent. North Korea in particular has no intention of allowing its enemies to fight the purely conventional war of their dreams.

Deterrence, both nuclear and conventional, cuts against the grain of traditional military thinking in the United States. Perhaps understandably, defense policy-makers have never fully internalized the ideas of the academic strategists. The core business of a military establishment, after all, is to prepare for war — whether or not it helps to preserve peace.

But in the nuclear era, there is more to national defense than just a good offense. As long as there is no reliable way to stop every possible use of nuclear weapons, nuclear-armed adversaries have no realistic choice but a strategy of deterrence, for however long the confrontation lasts. We have already faced a nuclear-armed North Korea for over a decade’s time, so we can only hope that our current crop of leaders will come to appreciate that point sooner rather than later.

Joshua H. Pollack is a senior research associate at the Middlebury Institute of International Studies at Monterey and editor of the Nonproliferation Review. He previously served as a consultant to the U.S. Department of Defense.