The Making of Central and Eastern Europe, London, Polish Research Centre, 1949.
The Idea of Apostolicity in Byzantium and the Legend of the Apostle Andrew, Cambridge, Mass., Harvard University Press, 1958.
The Slavs: Their Early History and Civilization, Boston, American Academy of Arts and Sciences, 1956.
The Ecumenical Councils, New York, Hawthorn Books, 1961.
The Slavs in European History and Civilization, New Brunswick, N.J., Rutgers University Press, 1962.
Byzance et la primauté romaine, Paris, Éditions du Cerf, 1964.
Early Christian and Byzantine Political Philosophy, Washington: Dumbarton Oaks Center for Byzantine Studies, 1966. (2 volume)
Les Slaves: histoire et civilisation: de l'Antiquité aux débuts de l'époque contemporaine, Paris: Seuil, 1970.
Byzantine Missions among the Slavs, New Brunswick, N.J., Rutgers University Press, 1970.
Photian and Byzantine Ecclesiastical Studies, London, Variorum Reprints, 1974.
Origins of Intelligence Services: The Ancient Near East, Persia, Greece, Rome, Byzantium, the Arab Muslim Empires, the Mongol Empire, China, Muscovy, New Brunswick, N.J., Rutgers University Press, [1974].