1945    Manhattan Project: the Atomic Age begins when the United States successfully detonates a plutonium-based test nuclear weapon at the Trinity site near Alamogordo, New Mexico.

1945    World War II: Hiroshima is devastated when the atomic bomb "Little Boy" is dropped by the United States B-29 ''Enola Gay''. Around 70,000 people are killed instantly, and some tens of thousands die in subsequent years from burns and radiation poisoning.

1945    World War II: Nagasaki is devastated when an atomic bomb, ''Fat Man'', is dropped by the United States B-29 ''Bockscar''. 39,000 people are killed outright.

1945    The USSR's nuclear program receives plans for the United States plutonium bomb from Klaus Fuchs at the Los Alamos National Laboratory.

1952    The United Kingdom successfully tests a nuclear weapon.

1953    Cold War: U.S. President Dwight D. Eisenhower formally approves the top secret document National Security Council Paper No. 162/2, which states that the United States' arsenal of nuclear weapons must be maintained and expanded to counter the communist threat.

1955    Eighteen Nobel laureates sign the Mainau Declaration against nuclear weapons, later co-signed by thirty-four others.

1962    Cuban Missile Crisis: US President John F. Kennedy, after internal counsel from Dwight D. Eisenhower, announces that American reconnaissance planes have discovered Soviet nuclear weapons in Cuba, and that he has ordered a naval "quarantine" of the Communist nation.

1967    More than sixty nations sign the Outer Space Treaty banning nuclear weapons in space.

1971    Eighty-seven countries, including the US, UK, and USSR, sign the Seabed Treaty outlawing nuclear weapons in international waters.