Simmons then served as a Military Attache to Spain from 1928 until 1931, whereupon he then commanded the 2nd Battalion, Cameron Highlanders, from 1932 to 1936. He then commanded the Southern Brigade in British mandated Palestine, as a lieutenant colonel, accompanied by his wife, and was Chief Staff Officer to British Forces in Palestine and Transjordan, from 1937 until 1939, during the Arab revolt in Palestine.