The atomic bomb dropped on Nagasaki was assembled on the Tinian Island of the Northern Mariana Islands on August 6, the same day as the first atomic bomb was dropped on Hiroshima. On August 8, the order was issued to drop the second bomb on the 9th, the following day, on either Kokura as the primary target or Nagasaki as the secondary target. On the morning of August 9th, the B-29 bomber called "Bockscar" reached the sky over Kokura but the view was bad because of the large amount of smoke and haze caused by the air raid conducted in Yahata the previous day. Therefore, they abandoned the primary target and flew to the secondary target, Nagasaki, where they dropped the atomic bomb.
Why was Nagasaki chosen as the one of the targets for the atomic bombing? It is believed that the main reason was because Nagasaki, with less damage caused by general air raids, was suitable for investigating and measuring accurately, atomic power. And it was also because military facilities such as shipbuilding companies and munitions factories were concentrated in Nagasaki.
But we knew later that the primary dropping point ordered by the US Army had been in the vicinity of the commercial center of Nagasaki city between Nigiwai bridge and Tokiwa bridge crossing over the Nakashima river which runs through the central part of Nagasaki.
