Complex sentence: He would then serve as Assistant Professor of Classics there until the United States entered World War II, during which time he served the Office of War Information in Syria and Lebanon and in 1945 became Director General of Antiquities of the Republic of Syria.
Split into simpler sentences: He would then serve as Assistant Professor of Classics there until the United States entered World War II, during which time he served the Office of War Information in Syria and Lebanon. In 1945 he became Director General of Antiquities of the Republic of Syria.