Passage: Shimomura was a native of Kōchi Prefecture, and a graduate of the 20th class of the Imperial Japanese Army Academy in 1908. His classmates included Prince Asaka, Prince Higashikuni Naruhiko, Prince Kitashirakawa Naruhisa, Mitsuru Ushijima, Heitarō Kimura, Takashi Sakai, and Shōjirō Iida. He subsequently graduated from the 28th class of the Army Staff College in 1916. After serving in a number of staff and administrative positions within the Imperial Japanese Army General Staff, Shimomura was posted to France as a military attaché in 1919. He returned to the Strategy and Planning bureau of the General Staff in 1921. He participated as part of the Japanese delegation to the Geneva Naval Conference negotiations from 1928–1929 and in 1931. From 1933-1935 he was commander of the IJA 1st Heavy Field Artillery Regiment.In 1935, Shimomura was assigned to the staff of the Kwantung Army and promoted to major general in 1936. He returned to the General Staff as Chief of the 4th Bureau from 1936–1937, and of the 1st Bureau from 1937-1938. He was strong proponent of a more aggressive approach towards the Kuomintang government in the Shanghai area and his recommendations influenced the decision of the Japanese General Staff to authorize the landings of the Japanese Tenth Army at the start of the Battle of Shanghai.Shimomura was promoted to lieutenant general in 1939. He was Commandant of the Artillery School in 1940, and given a field command in 1942 in the form of the Japanese Thirteenth Army based in Shanghai and surrounding provinces primarily as a garrison force to deter the possible landings of the Allies of World War II in the lower Yangtze River area of east central China.

Question: What department did he return to in 1921?
Strategy and Planning bureau of the General Staff