Thus far the chief purpose of our military establishment has been to win wars,” the American nuclear strategist Bernard Brodie wrote in 1946. “From now on its chief purpose must be to avert them.” Brodie’s injunction summed up the grim lesson of the first five decades of the twentieth century: after two horrific world wars and the development of nuclear weapons, it was clear that the next major conflict would produce no winners—only survivors. As U.S. President John F. Kennedy put it a decade and a half later, in the midst of the Cuban missile crisis, “Even the fruits of victory would be ashes in our mouth.” For decades, U.S. policymakers followed Brodie’s and Kennedy’s lead, putting deterrence—preventing rivals from attacking in the first place—at the center of U.S. defense strategy. [...]