Under a plan made by Koiso and others, the Kwantung Army in January 1933 began to attack Rehe, and negotiated the Great Wall in May to intrude into Huairou, Miyun and Pinggu, all in the suburbs of Beijing. Koiso insisted that the army should continue advancing to capture Peiping and Tianjin. Finally, the Kwantung Army signed the Tanggu Agreement with the National Government, forcing the latter to accept the Great Wall as the boundary with the puppet Manchukuo. Since then, political and military infiltrations into north China prepared the ground for Japan to start an all-out aggression war in the Peiping-Tianjin area.
In 1934 Koiso became General Officer commanding the 5th Division. The following year he was promoted senior general and appointed Commander in Chief of the Chosen Army stationed in Korea, where he gained the nickname "The Tiger of Korea." After the Lugouqiao Incident in 1937, he energetically supported Japan to unleash an all-out aggression war against China.
Succeeding Hideki Tojo as Prime Minister
Koiso retired from the army in 1938 and then became Minister of Overseas Affairs in Hiranuma and Mitsumasa"s cabinets respectively, and Governor-General of Korea in 1942. With the downfall of Hideki Tojo"s cabinet in 1944, Koiso was chosen to serve as the new Prime Minister. Although at that time, Japan was already in an unfavorable position in the war, he refused to take the defeat lying down, and continued pursuing a war policy.
The protracted war had depleted Japan of its natural resources. Meanwhile, its military strength was exhausted too. The possibility was virtually non-existent for it to win the war. Under such circumstances, Koiso dreamed that Japan could score a victory, and then would seize the opportunity to begin peace talks.
In an desperate effort to have the war situation under control, he set up a "Supreme War Guidance Council" comprising prime, war, naval and foreign ministers and chief of the general staff, to replace the original joint conference of the General Headquarters and the government.
At the August 9 Supreme War Guidance Council attended by the Emperor, Koiso specified the fundamental war guidance policy: "1. Mustering forces to defeat the enemy; 2. Completing the war; 3. Conducting operations overseas and turning the tables on the enemy." He reposed his hope in a planned decisive battle in the Philippines with the code name of "Tennouzan." Nevertheless, as Japanese forces suffered multiple defeats in the Philippines and the Taiwan Straits, the "Tennouzan Battle" didn"t actually happen.
The military failure forced Koiso to resort to diplomatic means to clear up the mess. After consulting with foreign, war and naval ministers, he decided in February 1945 to call Miao Bin of the puppet Nanjing government to Tokyo, hoping to get in touch with Chongqing"s National Government through Miao and then to become reconciled with the Soviet Union through Chongqing to end the war. Koiso indeed met with Miao in March, but his hopes turned out to be but wishful thinking. The USSR informed Japan in April that it would not extend the USSR-Japan Neutrality Pact. Koiso eventually resigned in 1945, prior to Japan"s utter defeat.
The postwar International Military Tribunal for the Far East condemned him to life imprisonment as Class-A war criminal. He died on November 3, 1950 at Sugamo Prison while serving his sentence.