High Quality Content by WIKIPEDIA articles! The Southern Expeditionary Army was a army group of the Imperial Japanese Army during World War II. It was responsible for all military operations in South East Asian and South West Pacific campaigns of World War II. The Southern Expeditionary Army Group was formed on November 6, 1941, under the command of Marshal Hisaichi Terauchi, with orders to attack and occupy Allied territories and colonies in South East Asia and the South Pacific.