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. He established his headquarters in Saigon and prepared for an invasion of Thailand. In December 1941, Iida took command of the newly formed Japanese Fifteenth Army, consisting of the IJA 33rd Division and IJA 55th Division. Beginning on 8 December, his forces easily overcame light Thai resistance, forcing Thailand into a mutual defense pact with Japan. On 20 January 1942, Iida's divisions crossed into Burma in the highly successful beginnings of the Burma Campaign.