By 1945, the United States nuclear fission program (the Manhattan Project) was very advanced. The first military usage of the new technology from the program occurred at Hiroshima, Japan on August 6, 1945.
The plane carrying the bomb, Enola Gay, flew high over the city and released its payload, a nuclear fission bomb named Little Boy. After hitting the ground, the bomb sustained a nuclear fission reaction, releasing huge amounts of energy into the immediate area, instantly vaporizing houses, humans, and all that stood in its path.
The pilots of the Enola Gay described seeing blinding light fill their aircraft and the entirety of the ground below being obscured by the billowing mushroom-shaped column of smoke. Reportedly, the co-pilot could taste the fission as lead in the air.
An estimated 70,000 Japanese civilians were instantly killed by the explosion and approximately the same were also instantly injured. The tremendous levels of radiation caused radiation sickness in nearly everyone that survived the explosion, but were close to it. For generations after the explosion, radiation-related genetic mutations caused deaths and diseases in children born by those exposed to the radiation. Official estimates cited 200,000 deaths as a result of the bomb being dropped on Hiroshima.
The area of the destruction caused by the bomb was about 4 square miles. In this area an estimated 48,000 buildings were completely destroyed. More damage occurred from the fires generated by the explosion over the next several days.
The ethicality of such a devastating attack on Japanese civilians by the United States military has been debated over and over. However, what is clear is that thousands of innocent lives were lost and many believe it was unnecessary, although it did greatly help the United States in defeating Japan in World War 2 and removing the militaristic government in power.
The economic impact of the attack required many decades for Japan to rehabilitate its economic stature in the world. Much of the rebuilding was financed by the United States government, which installed a democratic government in Tokyo, following the war. Since then, Japan has recovered quite well and now has one of the strongest economies in the world.