Kokura was the primary target for the Fat Man mission before Nagasaki.
On August 9, 1945, the B-29 Bockscar carried Fat Man toward Kokura, the location of a major Japanese munitions plant. The mission required a visual attack, but clouds and smoke obscured the city and its aiming point.
The crew made three bombing runs while fuel supplies declined. Smoke from fires caused by a large conventional raid on nearby Yahata the previous day contributed to the poor visibility. With Japanese defenses becoming more dangerous and fuel running low, the aircraft turned toward its secondary target, Nagasaki.
A last-minute break in the clouds over Nagasaki allowed the crew to release Fat Man visually. The mission is the source of the expression “Kokura’s luck,” although that phrase became popular only later and simplifies a complex combination of weather, smoke, fuel, and operational decisions.