Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity on July 16, 1945, and the first detonations of nuclear devices since the atomic bombing of Nagasaki on August 9, 1945. The purpose of the tests was to investigate the effect of nuclear weapons on warships.

The Crossroads tests were the first of many nuclear tests held in the Marshall Islands and the first to be publicly announced beforehand and observed by an invited audience, including a large press corps. They were conducted by Joint Army/Navy Task Force One, headed by Vice Admiral William H. P. Blandy rather than by the Manhattan Project, which had developed nuclear weapons during World War II. A fleet of 95 target ships was assembled in Bikini Lagoon and hit with two detonations of Fat Man plutonium implosion-type nuclear weapons of the kind dropped on Nagasaki in 1945, each with a yield of 23 kilotons of TNT (96 TJ).

The first test was Able. The bomb was named Gilda after Rita Hayworth's character in the 1946 film Gilda and was dropped from the B-29 Superfortress Dave's Dream of the 509th Bombardment Group on July 1, 1946. It detonated 520 feet (158 m) above the target fleet and caused less than the expected amount of ship damage because it missed its aim point by 2,130 feet (649 m).

The second test was Baker. The bomb was known as Helen of Bikini and was detonated 90 feet (27 m) underwater on July 25, 1946. Radioactive sea spray caused extensive contamination. A third deep-water test named Charlie was planned for 1947 but was canceled primarily because of the United States Navy's inability to decontaminate the target ships after the Baker test. Ultimately, only nine target ships were able to be scrapped rather than scuttled. Charlie was rescheduled as Operation Wigwam, a deep-water shot conducted in 1955 off the coast of Mexico (Baja California).

Bikini's native residents were evacuated from the island on board the LST-861, with most moving to the Rongerik Atoll. In the 1950s, a series of large thermonuclear tests rendered Bikini unfit for subsistence farming and fishing because of radioactive contamination. Bikini remains uninhabited as of 2017^[update], though it is occasionally visited by sport divers.

Planners attempted to protect participants in the Operation Crossroads tests against radiation sickness, but one study showed that the life expectancy of participants was reduced by an average of three months. The Baker test's radioactive contamination of all the target ships was the first case of immediate, concentrated radioactive fallout from a nuclear explosion. Chemist Glenn T. Seaborg, the longest-serving chairman of the Atomic Energy Commission, called Baker "the world's first nuclear disaster."

The first proposal to test nuclear weapons against naval warships was made on August 16, 1945, by Lewis Strauss, future chairman of the Atomic Energy Commission. In an internal memo to Secretary of the Navy James Forrestal, Strauss argued, "If such a test is not made, there will be loose talk to the effect that the fleet is obsolete in the face of this new weapon and this will militate against appropriations to preserve a postwar Navy of the size now planned." With very few bombs available, he suggested a large number of targets widely dispersed over a large area. A quarter century earlier, in 1921, the Navy had suffered a public relations disaster when General Billy Mitchell's bombers sank every target ship the Navy provided for the Project B ship-versus-bomb tests. The Strauss test would be designed to demonstrate ship survivability.

In August 1945, Senator Brien McMahon, who within a year would write the Atomic Energy Act and organize and chair the Congressional Joint Committee on Atomic Energy, made the first public proposal for such a test, but one designed to demonstrate the vulnerability rather than survivability of ships. He proposed dropping an atomic bomb on captured Japanese ships and suggested, "The resulting explosion should prove to us just how effective the atomic bomb is when used against the giant naval ships." On September 19, the Chief of the United States Army Air Forces (USAAF), General of the Army Henry H. Arnold, asked the Navy to set aside 10 of the 38 captured Japanese ships for use in the test proposed by McMahon.