Hiroshima and Nagasaki were big manufacturing and military strongholds in the Second World War
America had already bombed Tokyo so badly there wasn’t much left to bomb there. Hiroshima, which was largely untouched by the US air forces fire bombing campaign thus far, was the centre of Japanese shipping and held large military supply depots at the time. Nagasaki was the home to Mitsubishi Heavy Industries one of the main centres of steel and arms manufacture, and the dockyards at Nagasaki were the site for Japanese Navy’s ship building factories. Both cities represented major strategic targets for the Allies during the war and hence became the sites of the Atomic Bombing campaign.