Koiso resigned as premier after the start of the Battle of Okinawa, and following Japan's surrender he was convicted as a Class A war criminal and sentenced to life imprisonment, dying in prison in 1950.

Reassigned to the Kwantung Army General Staff in September 1912, Koiso was promoted to major and Battalion Commander of the 2d Infantry Regiment in August 1914.

After his promotion to colonel in February 1922, he was sent as a military attaché to Europe in June 1922, returning to assume command of the IJA 51st Division in August 1923.

[2] Koiso continued his predecessor's hardline policies, such as imposing the highly unpopular universal military conscription on Koreans, which forced them to fight for the Japanese, on August 1, 1943.

"[citation needed] After the Allied landing in Normandy (June 6) and the successful capture of Saipan (9 July), Hideki Tojo resigned and a new cabinet was formed.

The Army strongly favored General Hisaichi Terauchi; however, they could not afford to recall him to Japan from his role as commander-in-chief of all Japanese forces in Southeast Asia.

Koiso was supported by two former prime ministers, Mitsumasa Yonai and Hiranuma Kiichirō, and as no consensus could be reached on a more suitable alternative, their arguments prevailed.

Koiso attempted to end army-navy rivalry by creating a Supreme Chief of Staff (最高幕僚長), but this was structured to favor the army, thus bitter opposition from the navy doomed the plan.

Late in 1944, Koiso still planned to send Prince Fumimaro Konoe on a peace mission to neutral countries, Switzerland and Sweden, but it came to nothing.

During the remainder of Koiso's premiership in office, Japanese forces continued to suffer a string of defeats on all fronts at the hands of the Allies.

"Furthermore, despite the fact that Kuniaki Koiso was not directly responsible for the war crimes committed by the Japanese Army, he took no measures to prevent them or to punish the perpetrators when, as Prime Minister, it was within his power to do so.