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.

Applied effectively, deterrence discourages an adversary from pursuing an undesirable action. It works by changing the adversary’s calculation of costs, benefits, and risks. A country can, for instance, convince its opponents that an attack is so unlikely to succeed that it is not even worth the attempt: deterrence through denial. Or a country may convince its opponents that defeating it would be so costly as to be a victory in name only: deterrence through punishment. In either case, a rational adversary will decide to stay put.

Through the threat of denial or punishment, deterrence has helped keep the peace among major powers for over seven decades. Even 30 years after the end of the Cold War, it remains at the heart of U.S. defense strategy. The 2018 National Defense Strategy, for instance, begins by declaring that “the Department of Defense’s enduring mission is to provide combat-credible military forces needed to deter war and protect the security of our nation.”

By now, that declaration has been made so many times, over so many decades, that it has become an article of faith. Like several of its recent predecessors, the Trump administration has spent little time explaining exactly how the United States intends to deter existing and future rivals. The assumption is that it needs no explaining: modern weapons are so destructive that no sane leader would risk igniting a general war—and so the requirements for deterrence are relatively modest.

But such confidence is profoundly misplaced. In fact, deterring aggression has become increasingly difficult, and it stands to become more difficult still, as a result of developments both technological and geopolitical. The era of unprecedented U.S. military dominance that followed the Cold War has ended, leading to renewed competition between the United States and two great revisionist powers, China and Russia. Military competition is expanding to several new domains, from space and cyberspace to the seabed, and new capabilities are making it harder to accurately gauge the military balance of power. Meanwhile, advances in cognitive science are challenging the theoretical underpinnings of deterrence by upending our understanding of how humans behave in high-risk situations—such as when facing the possibility of war.