Kokura was the primary target before the Nagasaki mission diverted there in August 1945.
On August 9, 1945, the B-29 Bockscar approached Kokura with the Fat Man atomic bomb. Thick cloud and smoke prevented the crew from clearly identifying the aiming point. Because the mission required a visual bombing run, the aircraft moved to its secondary target, Nagasaki.
Nagasaki was also partly obscured, but the crew found a break in the clouds and released the bomb. The weapon exploded over the Urakami Valley, rather than directly over the city's central commercial district. The terrain and location affected the pattern of destruction, although the bombing caused enormous loss of life and damage.
Kokura's near-miss became a lasting historical reference. The city is sometimes mistakenly identified as the target that was bombed, while Hiroshima was the target of the earlier mission on August 6. The distinction matters because the two missions involved different aircraft, weapons, and target-selection circumstances.