Seventy years ago, in August 1945, the United States destroyed Hiroshima and Nagasaki with atomic weapons. Since then, neither the United States nor any other nuclear-armed country has used such weapons against an adversary. Why not? During the Cold War, Washington and Moscow made nuclear threats that could have led to catastrophe, and the history of US strategy and policy since 1945 includes many examples of risky nuclear bluster. But it also includes a number of cases in which decision-makers stopped short of nuclear use because of constraints against first use. These constraints are known collectively as the “nuclear taboo," an informal but widely-observed prohibition made up of moral, political, bureaucratic, military, practical, and diplomatic hurdles and inhibitions. This taboo has played an important role in preventing nuclear war. But even though it persists today, it may not be enough to prevent nuclear war in the future.

When the United States dropped atomic bombs on Japan in 1945, top US officials mainly thought of the weapon as a gigantic explosive device with powerful blast effects and were unaware of other dangers, such as radiation poisoning and fallout hazards. But the terrible effects of the weapon and the massive civilian casualties soon made it evident that the bomb was unique in other foreboding ways. Belatedly recognizing what the United States had wrought, President Harry Truman stopped the atomic bombings as Japan was preparing to surrender, telling his cabinet that the “thought of killing another 100,000 people was too horrible” and that he did not like the idea of killing “all those kids.”

The atomic bombings had a tremendous impact on international opinion and produced constraints against nuclear use, which US government officials had to take into account. Although US military leaders considered using nuclear weapons against North Korean and Chinese forces, they were unable to identify practical military targets, and global public opinion by that point constrained their use. In November 1950, for example, as US-led United Nations forces retreated in the face of Chinese entry into the Korean War, Truman administration officials debated the merits of attacking China with atomic weapons without UN approval. State Department planning adviser John K. Emmerson objected, arguing in a memo to Dean Rusk (who would later become secretary of state) that “the moral position of the United States would be seriously damaged.” Such action “would be deplored and denounced by a considerable number of nations who had up to that time supported the action in Korea. ... Should the next atomic bomb be dropped on an Asiatic population, it is easy for foresee the revulsion of feeling which would spread throughout Asia.”

Dismayed by the civilian casualties caused by atomic bombs in Japan and later by the incomprehensible destructiveness of hydrogen bombs, as demonstrated by US and Soviet thermonuclear tests, Truman had come to regard both as “weapons of last resort.” But his successor Dwight D. Eisenhower thought otherwise, believing that nuclear weapons could be used "just exactly as you would use a bullet or anything else,” as he said at a 1955 news conference. After the Korean War ended, Eisenhower and Secretary of State John Foster Dulles claimed they had secretly and successfully brought China to terms at the negotiating table by threatening to use nuclear weapons. In 1954, during the French War in Indochina, Eisenhower and Dulles considered using atomic bombs to break the Viet Minh siege at Dien Bien Phu. They also issued public nuclear threats and placed nuclear (as well as conventional) forces on high readiness during the Taiwan Strait crises of the 1950s. During the 1958 civil war in Lebanon between pro-Western Christians and Soviet-endorsed pan-Arab Muslims, Eisenhower put US nuclear forces on their largest and highest alert yet, with hundreds of strategic bombers readied for launch.

Yet even as Soviet nuclear forces grew and as Eisenhower and Dulles became more aware of nuclear dangers, their thinking shifted. During the 1958 Taiwan Strait Crisis, Eisenhower kept tight control over nuclear weapons and conceded in a message to the British foreign secretary that when you use nuclear weapons “you cross a completely different line.” Eisenhower came to view these weapons as belonging in a special category—although he continued to profess support for their use in the event of a Soviet onslaught against Western Europe or an attack against US territory. ...