The 109th Division was reformed in May 1944 from the 1st Independent Mixed Brigade and other Bonin Islands garrisons reinforced by recruits from the Kōfu mobilization district.

The southernmost cluster known as the Mother Island (group) of Haha-jima, Ane-jima, Imōto-jima, Mei-jima, along with the smaller Hira-shima and Muko-jima.

[5] The 2700 man 145th infantry regiment under Colonel Masuo Ikeda, a part of 46th division, was diverted from Saipan to Iwo Jima and Rear Admiral Toshinosuke Ichimaru that arrived with 2300 navy personnel.

[6] Many of the 109th's personnel had previously served in China, Burma and the Dutch East Indies and had become reservists who were now reluctant to go back to battle.

[5] To bolster morale the Japanese Emperor Hirohito sent his own palace guard of anti-aircraft gunners to Chichi Jima to support the troops[5]—as it was expected the main force of Americans would strike there, however the Allied Joint Chiefs of Staff decided to bypass Chichi Jima altogether.

Kuribayashi ordered his troops to inflict maximum damage and casualties on the enemy and to "kill 10 men before dying".

5200 men Brigade Artillery Group, Colonel Chosaku Kaido Naval Guard Force, Captain Samaji Inoue The exact strength of Navy personnel is subject to question.

[5] Kuribayashi—who had spent much time in Canada as deputy military attaché and traveled often in America—was in agreement, however they knew that at 628 miles (1,011 km) from Tokyo, Iwo Jima would be "a dagger aimed straight at the heart of the homeland".