With the start of the Second Sino-Japanese War, Iida became Chief of Staff of the Japanese First Army in China in 1938.

[2] With the start of the Pacific War, Iida was transferred south to take command of the Japanese 25th Army in Japanese-occupied French Indochina in 1941.

[3] Iida's 35,000 men quickly outmaneuvered British forces despite inclement terrain and limited supplies.

By May, British and Chinese forces in Burma had been driven back to India and China with some 30,000 casualties, against Japanese losses numbering only 7,000.

However, in 1945, Iida was recalled back to service to take command of the Japanese 30th Army in Manchukuo just before the Soviet invasion.