On September 23, in a brief written statement distributed to reporters, President Harry S. Truman revealed the chilling fact that would dominate American foreign policy for the remainder of the century: “We have evidence that within recent weeks an atomic explosion occurred in the U.S.S.R.”

By itself the development was not unexpected. Everyone had assumed the Soviets would build their own bomb eventually; the worrisome part was the timing. Although the scientific principles behind the atomic bomb had been widely known for a decade, the engineering details of America’s nuclear arsenal were a tightly guarded secret. Experts had predicted a Soviet bomb around 1952. The achievement of a successful explosion so much earlier meant one of two things: Either the Soviets were a lot smarter than we had thought or they had spies in America’s laboratories.

The initial reaction from officials, press, and public was calm. Almost since Hiroshima, Americans had been reading lurid predictions of nuclear war, and after four years of scaremongering, news of a mere test explosion came as something of an anticlimax. On the other hand, it did mean that all those scenarios of horror were now real and immediate. In the fencestraddling style that had recently lost him the Presidency, Gov. Thomas Dewey of New York said there was “not the slightest reason for hysteria or fright” but warned of a possible “atom bombing of New York today, next month, or next year.”

Otto Hahn, the German scientist who had co-discovered nuclear fission, called Truman’s announcement “good news.” “If both the United States and Russia have it, there will be no war,” he said. “It will be the same as it was with poison gas.” The physicist Harold Urey, who had made important advances in isotopic separation, had a gloomier prognosis: “There is only one thing much worse than one nation having the atomic bomb, and that is to have two nations possessing it.” Both these Nobel laureates turned out to be right in a way.

While there was no nuclear war, the United States and the U.S.S.R. both sped up development of the much deadlier hydrogen bomb, along with ways to deliver it with evergreater accuracy over ever-longer distances. Before long each side had the ability to annihilate the other many times over by remote control. Even after a painfully slow mutual withdrawal from the precipice, the effects of that terrible struggle remain with us today.

In retrospect, America overestimated the Soviets’ readiness to use their atomic arsenal. The fundamental and obvious truth about nuclear weapons —that they can never be set off without eliciting an equally horrific retaliation—was as well understood in the Kremlin as anywhere else. Unfortunately, until at least the 1970s, the Soviets gave Americans little reason to assume that they understood this. And while idealists spoke of banning or controlling nuclear weapons with global agreements, the Soviets’ record of international cooperation did not justify betting world peace on their trustworthiness.

Most prescient of all, perhaps, were the remarks of