During World War I, Hiroshima became a focal point of military activity, as the Japanese government entered the war on the Allied side. About 500 German prisoners of war were held in Ninoshima Island in Hiroshima Bay.
The growth of Hiroshima as a city continued after the First World War, as the city now attracted the attention of the Catholic Church, and on May 4, 1923, an Apostolic Vicar was appointed for that city. During World War II, the 2nd General Army and Chugoku Regional Army were headquartered in Hiroshima, and the Army Marine Headquarters was located at Ujina port. The city also had large depots of military supplies, and was a key center for shipping.
The bombing of Tokyo and other cities in Japan during World War II caused widespread destruction and hundreds of thousands of deaths. For example, Toyama, an urban area of 128,000, was nearly fully destroyed, and incendiary attacks on Tokyo claimed the lives of 100,000 people. There were no such air raids in Hiroshima. However, the threat was certainly there and to protect against potential firebombings in Hiroshima, students (between 11–14 years) were mobilized to demolish houses and create firebreaks.
On Monday, August 6, 1945, at 8:15 a.m., the Atomic Bomb "Little Boy" was dropped on Hiroshima by an American B-29 bomber, the Enola Gay, flown by Paul Tibbets, directly killing an estimated 80,000 people. By the end of the year, injury and radiation brought total casualties to 90,000–140,000. The population before the bombing was around 340,000 to 350,000. Approximately 69% of the city's buildings were completely destroyed, and another 7% severely damaged.
Research about the effects of the attack was restricted during the occupation of Japan, and information censored until the signing of the San Francisco Peace Treaty in 1951, restoring control to the Japanese.
The oleander is the official flower of the city of Hiroshima because it was the first to bloom again after the explosion of the atomic bomb in 1945.
http://m.youtube.com/watch?v=AtSt5XZ7fq4 bomb video
Government documents (notably the report of the US Strategic Bombing Survey released in July, 1946) and memoirs of military and civilian leaders like Dwight Eisenhower, Douglas MacArthur, Chester Nimitz and Chief of Staff Adm Leahy would be a good starting point. They will explain to you why the bombs were not necessary to bring about the Japanese surrender and how they did not save American (or Allied or Japanese lives).
In the months before Little Boy was dropped, the US had destroyed 67 Japanese cities and killed some 2 million Japanese civilians by convention TNT and incendiary bombs. Curtis LeMay complained that he had no meaningful targets left against which to task his bombers. Military planners had concluded that Operation Downfall, the invasion of Japan, was not, in all probability, going to be necessary. The earliest the landings were going to happen, if at all, was early November. The landings were not going to be anywhere near Nagasaki (never intended as the primary target of Fat Man in any case) or Hiroshima.
After the fall of Saipan, the Tojo Cabinet fell and the peace movement that had been trying to end the war gained strength. Peace overtures were sent through Sweden, Switzerland, Moscow and elsewhere.