The primary target of the 9 August 1945 atomic-bomb mission was Kokura.
Kokura, a major arsenal and industrial center in northern Kyushu, was listed ahead of Nagasaki in the mission plan. The B-29 Bockscar made several attempts to locate the aiming point, but visibility was poor.
Smoke from a conventional raid on nearby Yahata, combined with cloud cover, obscured much of Kokura. Because the aircraft was carrying limited fuel, the crew could not continue circling indefinitely. They diverted to Nagasaki, the secondary target.
This episode produced the expression “Kokura's luck,” describing how weather and smoke spared the city from the atomic attack. The phrase is a later popular summary, not an official wartime designation.