Kokura was the primary target of the atomic-bomb mission that became the Nagasaki attack.
On August 9, 1945, the B-29 Bockscar carried Fat Man toward Kokura, a city in northern Kyushu that contained an important arsenal. When the crew arrived, cloud cover and smoke obscured the aiming point. Orders required visual bombing, so the aircraft could not safely release the weapon over the city.
After several passes and worsening fuel problems, the crew diverted to Nagasaki, the secondary target. A break in the clouds allowed bombardier Kermit Beahan to release Fat Man. The bomb exploded over the Urakami Valley shortly after 11 a.m.
The phrase “Kokura’s luck” is often used for the weather-related change of target, although the mission also involved fuel limitations and difficult flight conditions. Kyoto had been considered during target planning but was removed from the final atomic-bomb target list.