Isogai was promoted to major general in 1933. In March 1936, he was appointed head of the Military Affairs Bureau of the Ministry of War, and he was promoted to lieutenant general in December. After the outbreak of the 1937–1945 Sino-Japanese War, Isogai was sent to China in August 1937 to command the 10th Infantry Division. He paired with Lieutenant General Itagaki Seishiro's 5th Infantry Division for an offensive along the Tianjin (Tientsin)-Shanghai railroad. The 10th Division was ambushed at Taierzhuang (T' ai-Erh-Chuang) in April 1938. Itagaki's division broke through and relieved Isogai but at a heavy cost in lives and prestige. Isogai took the blame for the debacle and never again held active combat command.
In June 1938, Isogai was appointed chief of staff of the Guandong (Kwantung) Army and participated in the Battle of Nomonhan/Khalhin-Gol in Mongolia that same year. In December, following the defeat of the Guandong Army by Soviet forces in that battle, Isogai was dismissed. In January 1942, he was appointed governor of Japanese-occupied Hong Kong. After the war, Isogai was tried as a war criminal at Nanjing (Nanking) in Jiangsu and was sentenced to life imprisonment on 22 July 1947. He was released in August 1952. Isogai died in Chiba on 6 June 1967. Kotani Ken
Coble, Parks M. Facing Japan: Chinese Politics and Japanese Imperialism, 1937–1941. Cambridge, MA: Harvard University Press, 1991.; Hsu, Long-hsuen, and Chang Ming-kai. History of the Sino-Japanese War, 1937–1945. Taipei: Chung Wu Publishing, 1991.