Complex sentence: When Italy joined the Allied of World War I he held his post to the General Staff but he was soon named commander of "Basilicata Infantry Brigade" then he assumed his duty as chief of the staff of First Army.
Split into simpler sentences: When Italy joined the Allies in World War I, he held his post on the General Staff, but he was soon named commander of the "Basilicata Infantry Brigade". Then he assumed his duty as chief of the staff of the First Army.