Sun teaches modern European history, the history of Germany, and World War II in Asia and the Pacific. His recent publications include Before the Enemy Is within Our Walls: A Social, Cultural, and Political History of Catholic Workers in Cologne, 1885–1912 (Brill 1999), "Catholic-Marxist Competition in the Working-Class Parishes of Cologne during the Weimar Republic" (Catholic Historical Review 83, No. 1 (January 1997):20-43), and "Arbeiter Priester und die 'Roten': Kulturelle Hegemonie im Katholischen Milieu, 1885–1933" in Thomas Mergel and Thomas Welskopp, eds., Geschichte zwischen Kultur und Gesellshaft: Beitrage zur Theoriedebatte (Munich: Beck Verlag, 1997).