The United States became the world's first #nuclear power on July 16, 1945, when it successfully detonated an atomic bomb at a testing site in New Mexico, as part of the Manhattan Project. Less than one month later, on August 6th and 9th respectively, the U.S. dropped atomic bombs (known as "Fat Man" and "Little Boy") on the Japanese cities of #Hiroshima and #Nagasaki; forcing the Japanese into surrender and bringing the Second World War to an end. These bombings remain the only nuclear attacks in history, however, advancements in nuclear technology and the threat of nuclear warfare have became a defining characteristic of post-WWII diplomacy, particularly during the #coldwar.

Four years after Hiroshima, the Soviet Union developed its first nuclear weapon (known as "First Lightning") and the Nuclear Arms Race began. By the mid-1960s, both the #US and #USSR had amassed enough nuclear warheads to annihilate any opponent; it was generally accepted that if one were to launch a nuclear attack against the other then a retaliatory "second strike" was guaranteed and this would result in "mutual assured destruction" (MAD).

The U.S.' nuclear arsenal reached its largest size in the mid-1960s, at over 31,000 warheads, before falling to the low 20,000s in the 1980s. The Soviet arsenal's growth was more gradual, but it overtook that of the U.S. in 1978, and it peaked at over 40,000 nuclear warheads in 1986. The #UK, #France, and #China also developed their own nuclear programs; while these did play an important psychological role in the Cold War, their nuclear programs were primarily focused on energy (France still gets a higher share of its electrical energy from nuclear power than any other country) and self-defense, and their arsenals paled in comparison to those amassed by the U.S. or USSR. Israel is also believed to have possessed nuclear weapons since the late 1960s, but has never openly acknowledged or denied this.