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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 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 become a defining characteristic of post-WWII diplomacy, particularly during the Cold War.

The Nuclear Arms Race

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 U.S. 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 concept of a nuclear triad was also established, where a wide enough network of armed aerial bombers, land-based missile silos, and submarines scattered across the globe ensured that a single attack would not disarm the enemy and a second strike was almost a certainty. This came to a head in 1962; the U.S. stored warheads in Italy and Turkey (its NATO allies), and the USSR retaliated by building silos in Cuba; the Cuban Missile Crisis then saw the world on the brink of nuclear war for over a month, but was eventually resolved through tense diplomacy, rather than official military action. 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.

Post-Cold War

The global stockpile of nuclear weapons peaked in 1986, before Soviet dissolution and the end of the Cold War saw a significant de-escalation in tensions between the east and west, and the process of nuclear disarmament began. Since the 1980s, Russia's nuclear arsenal has decreased by a factor of nine, and the U.S. arsenal is now six times smaller. Disarmament between the U.S. and Russia has occurred at a fairly similar pace, and the removal of weapons from the former-Soviet states of Belarus, Kazakhstan, and Ukraine were instrumental in their integration into the post-Soviet world. South Africa remains the only country to have established its own nuclear weapons program that was then dismantled. However, as these countries denuclearized, India and Pakistan developed their first nuclear weapons in the late 1990s, which they maintain as part of nuclear deterrent programs. North Korea has also developed a nuclear weapons program since the early 2000s, but its perceived hostility to the west and withdrawal from the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) has seen its nuclear tests heighten tensions with some neighbors and the U.S.. In 2023, it is believed that nine countries are in possession of nuclear weapons, with the U.S. and Russia in possession of several thousand each. Although these countries maintain that their nuclear stockpiles are kept in the interests of self-defense, nuclear testing or the mobilization of nuclear forces (such as in Russia in 2022) are generally interpreted as a sign of international aggression.