Operation Castle Bravo (or just Bravo) was a nuclear test conducted by the United States of America on March 1, 1954. It is notable for the massively higher yield than originally predicted (predicted 4 to 6 megatons, actual yield around 15mt), which made it the worst radiological accident ever caused by the United States.
The natural uranium used in the bomb produced heavy fallout. A change in the wind direction caused the fallout to happen in places where it caused trouble. Further because the yield was greater than expected the fallout was dispersed more widely.
In 1954 the Cold War was at its height. Communists in the Soviet Union were a threat to democracy. Preventing the USSR getting ahead of the Free world was seen as a high priority. Nuclear explosions were not well understood. Modern computer simulations of nuclear explosions were not then possible. All the above contributed to the accident.
Marshall Islands natives suffered birth defects after exposure to the radiation from the test. Japanese fishermen exposed to radiation became sick and one died. It is impossible to estimate what the costs would have been if the United States had been more cautious over this and other nuclear tests and had let the USSR get ahead of the West.
- Castle Bravo Wikipedia