 From 1939–1940, he served on the senior staff of the IJA 21st Army tasked with the Canton Operation, and from February 1940, on the senior staff of the IJA 22nd Army, a garrison force in Guangxi which fought at the Battle of South Guangxi and which was disbanded for its insubordination in invading French Indochina.Nakai was sent back to Tokyo, but returned later in 1940 as a member of the staff of the Indochina Expeditionary Army.After a brief stint as an instructor at the Toyama Army Infantry School, Nakai was assigned the post of chief-of-staff of the IJA 20th Division, based in Korea.