As described above, Hiroshima was one of the most important military cities in Japan. Hiroshima Castle became a military base. With the outbreak of the First Sino-Japanese War started in July 1894, Hiroshima became a major base for troop dispatchand military logistics. It was not generally known that a major poison gas production facility had been constructed (in April, 1929) on the Okunoshima Island in the Seto Inland Sea, located in the prefecture. In addition, during the final days of World War II, the Headquarters of the Second General Army, established to command troops in western Japan, was stationed in Hiroshima in preparation for decisive battles on the mainland.
On July 25, 1945, the United States narrowed down the potential targets of the A-bombing to the four cities of Hiroshima, Kokura, Niigata and Nagasaki (Kyoto was also picked at first, but excluded for political reasons). On August 2, the decision to use an atomic bomb on August 6 was made and the first target was Hiroshima. In the early dawn hours on August 6, 1945, the Enola Gay, a B-29 bomber, took off from Tinian Island in the Mariana Islands and headed straight to Hiroshima with an atomic bomb, nicknamed “Little Boy.” The morning of August 6, the weather in Hiroshima was sunny. Then, at 8:15 a.m., the Enola Gay dropped an atomic bomb. After falling for approximately 43 seconds, it exploded mid-air in a nuclear eruption approximately 600 meters above the Shima Hospital, slightly southeast of the Aioi Bridge which was the target. In the moment it exploded and a gigantic fireball appeared, intense heat rays were emitted, causing the ground temperature of the surrounding area reach to temperatures between 3,000 to 4,000 degrees centigrade. Also, the intensive bomb blast created maximum wind speeds of 440 meters per second at the hypocenter. The blast spread out radially, and swept the entire city in about 10 seconds.

Description of the damage from A-bombing to Hiroshima

Source:“ Hiroshima Pocket Peace Guide” (as of February 1, 2015) (Produced by Peace Promotion Division, International Peace Promotion Department, Citizens Affairs 2 Bureau, The City of Hiroshima)

Although the heat rays were only emitted for a short period of time, their extreme intensity caused anyone within a one-kilometer radius from the hypocenter either to die on the spot or to receive multiple severe burns. Even people over three kilometers away from the hypocenter were burned on the parts of their bodies that were not covered by clothing. Wooden structures located within two kilometers of the hypocenter were completely destroyed, and there were people left trapped under the remains. Shortly after, the fires broke out spontaneously from the heat rays, and the fires that started among the ruined buildings spread. Thirty minutes after the explosion, Hiroshima was covered in a disastrous firestorm. Anything flammable located within a two-kilometer radius of the hypocenter was engulfed in flames, and many people were burned to death.  

Radioactive waves emitted from the explosion caused severe damage to the human body. At a distance of one kilometer from the hypocenter, neutrons and gamma rays gave off four grays of radiation, estimated to be enough radiation to cause the death of one out of every two people. One after another, people showing no external injuries became sick after a couple of days and later died. People who were not directly exposed to the atomic bombing were also exposed to radiation. This included those who lived on the outskirts of town, where people were not directly exposed to the bombing but radioactive black rain fell, and those who were exposed to residual radiation upon entering the city.
It is unknown exactly how many people died from the atomic bombing. On August 6, at the time of the atomic bombing, it is estimated that there were about 350,000 people in Hiroshima City, including inhabitants, military personnel, and people who lived outside the city and commuted to the city for work. At that time, there were not only Japanese people, but also many people of different nationalities living in the city. They included Japanese-Americans born in the U.S., German priests, exchange students from Southeast Asian countries, people from Korean Peninsula and Taiwan which had both been under Japanese colonial rule at that time, people from the Chinese mainland, and a dozen U.S. prisoners of war. None of them escaped the devastation of the atomic bombing. The resources that Hiroshima City handed over to the United Nations in 1976 estimated the number of deaths to have been 140,000 ± 10,000 at the end of 1945.
Rescue operations, the disposal of the dead and the removal of debris were carried out immediately after the bombing by the military. People searched everywhere for their lost parents, children, and siblings. Many had to cremate the remains of family members on their own. As unprecedented chaos unfolded, Hiroshima was blanketed in a cloud of grief and loss. Signs of the city’s future reconstruction from the catastrophe were far, far, in the distance.