Kokura was the primary target before the 1945 bombing mission shifted to Nagasaki.
On August 9, 1945, the B-29 bomber Bockscar approached Kokura to deliver the plutonium bomb Fat Man. Thick cloud and smoke prevented the crew from identifying the aiming point clearly, so the aircraft proceeded to its secondary target, Nagasaki.
Nagasaki was not the mission’s first choice. Once the aircraft reached the city, a break in the clouds allowed the bombardier to make the visual attack required by the mission order. The bomb detonated over the Urakami Valley, an industrial area in northwestern Nagasaki.
Kokura’s experience produced the phrase “Kokura’s luck,” a later expression for narrowly avoiding disaster. The city’s survival was not caused by a cancellation of the mission; the aircraft simply moved to the preselected secondary target after weather and visibility interfered.