Perhaps more than at any time since the end of the Cold War, the Pentagon is getting serious about nuclear conflict.

Over the past few years, the administrations of U.S. Presidents Barack Obama and Donald Trump have reportedly studied how to respond if Russia fires off a nuke during a war with NATO in the Baltic region. U.S. Secretary of Defense Mark Esper recently participated in a war game simulating a limited nuclear conflict with Moscow. The U.S. Navy just deployed a submarine-launched, low-yield nuclear warhead designed for such a scenario. Critics have responded by calling these preparations provocative and dangerous.

The controversy is reviving hard questions about nuclear strategy and deterrence that U.S. officials last faced during the twilight of the conflicts against the Soviet Union. Looking back at that era reminds us that mastering the realities of geopolitical rivalry often requires embracing the apparent absurdities of nuclear statecraft.

Much of the current debate revolves around the Russian strategy known as “escalate to de-escalate.” It refers to a scenario in which Russia would use conventional forces to quickly seize some piece of NATO-held territory, such as a slice of Estonia. Moscow would then introduce nuclear weapons into the intensifying conflict — perhaps by firing off a demonstration shot or even by targeting NATO forces in the field — in hopes of deterring the alliance from retaking the conquered territory.

Experts debate whether escalate to de-escalate is really Russian doctrine, but it is clearly a possibility that NATO must contend with. This is why the Pentagon is rehearsing for limited nuclear war — to show that it can respond to Russian nuclear strikes in a proportional, and thus credible, manner that signals resolve without unleashing the apocalypse.

Yet this approach raises some sharp questions. Would the U.S. actually use nuclear weapons in a showdown with Russia? If so, could it keep a nuclear exchange limited? How much do perceptions of nuclear strength or weakness really matter? And why should the U.S. prepare to execute contingencies that carry an uncomfortably high possibility of disaster? Fortunately, we can get insight on these issues by looking back at the Cold War.

For decades, grappling with the intricacies of nuclear deterrence was a way of life for American planners, because the Pentagon rarely felt confident in NATO’s ability to check a Soviet assault by relying solely on conventional weapons. So the U.S. developed an evolving variety of nuclear strategies. Some rested on a near-instantaneous escalation to all-out nuclear war; others involved using calibrated nuclear strikes to signal American will and to shock Soviet leaders into de-escalation. These variations notwithstanding, four points stand out.

First, regardless of what they said in public, most policymakers were completely appalled by the thought of using nuclear weapons in a crisis. U.S. President Dwight D. Eisenhower, whose entire defense strategy hinged on waging pre-emptive nuclear war, constantly reminded his advisers that such a war might mean the death of civilization. U.S. Secretary of Defense Robert McNamara secretly recommended to the two presidents he served — John F. Kennedy and Lyndon Johnson — that they never wage nuclear war, under any circumstances. President Ronald Reagan was often vilified as a warmonger, but he ritualistically repeated the mantra, “A nuclear war cannot be won and must never be fought.” Even in war games, U.S. officials were incredibly reluctant to cross the nuclear threshold. In other words, two generations of leaders built strategies around a threat in which they fundamentally did not believe.

Second, a key reason U.S. officials were so repelled by nuclear war was their intense skepticism that it could be kept limited. The Kennedy administration might talk about developing discrete nuclear options for responding to conventional attacks, but its leaders worried that the consequences of using even a single nuclear weapon would be unpredictable and uncontrollable. “The line between non-nuclear war and nuclear war is distinct and observable,” McNamara said. “However, once the momentous decision has been made to cross that line, everything becomes much more confused.” McNamara’s successors, even those who labored to introduce limited nuclear options into U.S. strategy, mostly came to the same conclusion.

So was the entire Cold War arms race an exercise in futility? Not really, because a third point is that perceptions of the strategic balance still mattered enormously: They shaped risk-taking and decision-making. Eisenhower and Kennedy may have believed that a nuclear war would leave no winners, but the vast nuclear superiority America enjoyed in the late 1950s and early 1960s provided critical leverage in staring down Soviet challenges in Berlin and during the Cuban missile crisis.

As the nuclear balance shifted in the late 1960s and 1970s, it was Moscow that became more assertive, intervening in third-world hot spots and subtly intimidating Washington’s exposed European allies. When the balance shifted back in the 1980s, Soviet officials understood that America’s superiority gave it an edge in diplomatic crises, because Washington would enjoy a hard-to-quantify but undeniable military advantage if war came.

Finally, all this meant that U.S. leaders believed they had little alternative but to go down the rabbit hole — to craft strategies and doctrines that they desperately hoped never to carry out. Administration after administration developed plans for vaporizing the Soviet Union and its allies. In the late 1970s and 1980s, the Carter and Reagan administrations produced a cold-blooded strategy based on decapitating the Soviet leadership and waging a protracted nuclear war.

They didn’t do so because they thought it was feasible to fight such a conflict. Harold Brown, Jimmy Carter’s secretary of defense, admitted that he had “no illusion that a large-scale nuclear war” could be a “sensible, deliberate instrument” of policy. They did so because a demonstrated ability to target what the enemy most valued, to close off all of its paths to victory in a nuclear war, was the only way to make certain that the nuclear threshold was never crossed.

The circumstances today are different, of course. It was one thing to threaten nuclear war over Western Europe, which was critical to the global balance of power. It would be another thing to do so over a piece of territory in the Baltics. America’s best option, then, would be to further strengthen its conventional deterrence in Eastern Europe, so as to reduce the on-the-ground vulnerabilities that might make an escalate to de-escalate gambit attractive to Russia.

Yet the history of the Cold War also shows that preparing to fight a limited nuclear war over the Baltics may not be as insane as it seems. Perceptions of nuclear opportunity and danger matter enormously in great-power competition. Convincing the other side that America has the ability to execute even incredibly risky options may be the best way of ensuring that the U.S. never actually has to use them.

Hal Brands is a Bloomberg Opinion columnist, the Henry Kissinger distinguished professor at Johns Hopkins University’s School of Advanced International Studies, and a scholar at the American Enterprise Institute.

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