The United States launched the Nuclear Age in the pre-dawn hours of 16 July 1945 when it detonated a 20-kiloton atomic bomb code-named ”Trinity“ at Alamogordo, New Mexico.

Under the umbrella of the “Manhattan Project”, the test’s original purpose had been to confirm that an implosion-type nuclear weapon design was feasible. It also gave United States’ scientists and the military an idea of what the actual size and effects of such nuclear explosions would be before using them in combat.

While the Alamogordo test demonstrated many of the explosion's effects, it failed to provide a meaningful comprehension of radioactive nuclear fallout, which was not well understood by project scientists until years later.

The United States dropped two atomic bombs on Japan towards the end of World War II: one an untested gun-type fission bomb called “Little Boy” on Hiroshima on 6 August 1945; another implosion-type bomb tested at Alamogordo for the first time a month earlier and called “Fat Man” on Nagasaki on 9 August. Together these two bombs killed some 220,000 Japanese citizens outright, with over 200,000 more dying subsequently from lethal radiation overdoses.