Kokura was the primary target before the Nagasaki bombing mission switched targets.
On 9 August 1945, the B-29 Bockscar departed Tinian carrying the plutonium bomb Fat Man. The mission plan listed Kokura Arsenal in Kokura, Japan, as the primary target and Nagasaki as the secondary target. When the aircraft reached Kokura, clouds and smoke prevented the crew from making the required visual bombing run.
The smoke was partly linked to a major conventional air raid on nearby Yahata the previous day. With fuel becoming limited, the crew left Kokura and flew toward Nagasaki. A temporary break in the clouds enabled a visual release over the city's industrial valley.
The phrase “Kokura's luck” is often used to describe the city's escape from atomic destruction, although the outcome resulted from operational conditions rather than luck alone. The mission's target change is also why accounts of the Nagasaki bombing often mention Kokura first.
Nagasaki's destruction became part of the final sequence leading to Japan's surrender announcement on 15 August and the formal signing on 2 September 1945.