Operation Crossroads was a pair of nuclear tests conducted by the United States in 1946 at Bikini Atoll. They were the first tests of nuclear bombs since Trinity. The US government wanted to see the effects of a nuclear bomb on naval vessels, first from above, then from below. The 23 kiloton bombs Able and Baker were detonated at an altitude of 520 feet (158 meters) above the target and 90 feet (27 meters) underwater. A fleet of 95 vessels was assembled with varying distances from the target. The ship density of ~20 ships per square mile was three to five times greater than military doctrine would allow, but the goal was not simulation, but rather, to measure damage at different distance intervals. The fleet included four obsolete battleships, two aircraft carriers, two cruisers, eleven destroyers, eight submarines, numerous auxiliary and amphibious vessels, and three surrendered German and Japanese ships, including the battleship Nagato, flagship of Admiral Yamamoto during the attack on Pearl Harbor.

The bomb Baker was suspended beneath an amphibious assault ship, of which no identifiable piece was ever found.

To prepare for the tests, the 167 islands natives were evacuated, but due to radioactive contamination, Bikini Atoll remains uninhabitable.