Kokura was the primary target during the 9 August 1945 Fat Man mission before the crew diverted to Nagasaki.
The B-29 Bockscar carried Fat Man from Tinian with Kokura designated as the first target. The city contained an important arsenal, making it a major military-industrial objective.
When Bockscar reached Kokura, clouds, smoke, and haze prevented the bombardier from obtaining the required visual aiming point. The crew made three bomb runs over roughly 50 minutes, but fuel was becoming increasingly limited.
The aircraft therefore turned toward its secondary target, Nagasaki. A break in the clouds eventually allowed the bomb to be released over the city’s industrial valley. The phrase “Kokura’s luck” later became associated with the city’s escape from atomic bombing, although the diversion resulted from weather, smoke, fuel concerns, and mission requirements rather than luck alone.