Extended Deterrence
So the problem remained: How could the United States credibly defend its allies against superior Soviet (and, in Asia, Chinese) forces? The answer was the concept of extended deterrence. The Soviets knew that to attack the American heartland with nuclear arms would mean utter devastation. Under extended deterrence, the Americans would treat our allies as indistinguishable from ourselves, and we would defend Paris or Amsterdam as we would New York or Chicago. (You might see references to the “nuclear umbrella” as a description of how the U.S. arsenal protects friends beyond North America.)

Flexible Response
But what if the Soviets went ahead and marched into Europe instead of launching nuclear weapons? A madman’s threat to start blowing up Soviet cities in response—that is, to engage in massive retaliation—was ghastly and immoral. And from the point of view of deterrence, it was even worse: It wasn’t credible.

The answer to this dilemma in the 1960s was a NATO policy—one still in effect—called flexible response. During the Cold War, NATO was outgunned. It could not win a major conventional war in Europe against the U.S.S.R. Instead, the U.S. and NATO promised high risks of escalation. If you invade us, we told the Soviets, we’ll hold you off as long as we can with any number of conventional options. But we reserve the right to escalate the conflict—and even to use nuclear weapons first, if that’s what it takes to save ourselves and our allies.

If NATO, for example, were to face gigantic columns of armor, we reserved the right to strike that armor with tactical nuclear arms. If Soviet echelons were massing in rear areas, perhaps near the U.S.S.R. itself, we reserved the right to strike those echelons, even if it meant a wider war.