https://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki#Hiroshima
At the time of its bombing, Hiroshima was a city of both industrial and military significance. A number of military units were located nearby, the most important of which was the headquarters of
Field Marshal Shunroku Hata's
Second General Army, which commanded the defense of all of southern Japan,
[106] and was located in
Hiroshima Castle. Hata's command consisted of some 400,000 men, most of whom were on Kyushu where an Allied invasion was correctly anticipated.
[107] Also present in Hiroshima were the headquarters of the
59th Army, the
5th Division and the
224th Division, a recently formed mobile unit.
[108]The city was defended by five batteries of 7-cm and 8-cm (2.8 and 3.1 inch)
anti-aircraft guns of the 3rd Anti-Aircraft Division, including units from the 121st and 122nd Anti-Aircraft Regiments and the 22nd and 45th Separate Anti-Aircraft Battalions. In total, over 40,000 military personnel were stationed in the city.
[109]
Hiroshima was a minor supply and logistics base for the Japanese military, but it also had large stockpiles of military supplies.
[110] The city was also a communications center, a key port for shipping and an assembly area for troops.
[75] It was also the second largest city in Japan after Kyoto that was still undamaged by air raids,
[111] due to the fact that it lacked the aircraft manufacturing industry that was the XXI Bomber Command's priority target. On July 3, the Joint Chiefs of Staff placed it off limits to bombers, along with Kokura, Niigata and Kyoto.
[112]
The center of the city contained several
reinforced concrete buildings and lighter structures. Outside the center, the area was congested by a dense collection of small timber-made workshops set among Japanese houses. A few larger industrial plants lay near the outskirts of the city. The houses were constructed of timber with tile roofs, and many of the industrial buildings were also built around timber frames. The city as a whole was highly susceptible to fire damage.
[113]