Kokura was the primary target of the Fat Man mission before the bomber diverted to Nagasaki.
On August 9, 1945, Bockscar approached Kokura carrying the plutonium bomb Fat Man. The crew found the city obscured by cloud and smoke, including smoke associated with a conventional bombing raid on nearby Yahata the previous day.
The mission required a visual attack, so Bockscar made several attempts to identify the aiming point. With fuel running low, pilot Charles Sweeney and weaponeer Frederick Ashworth decided to proceed to the secondary target, Nagasaki.
A last-minute opening in the clouds over Nagasaki allowed the crew to drop the bomb. Kokura’s experience later inspired the expression “Kokura’s luck,” although that phrase is a retrospective popular description rather than an official wartime term. Kyoto was excluded from the atomic target list, while Yokohama was a conventional bombing target.