He became a fellow of New College after completing a thesis on modern Spanish history but subsequently specialised in medieval history.

Selected publications

* The origins of military power in Spain, 1800-1854. Oxford University Press, Oxford, 1967. (Oxford Historical Monographs)
* The Northern Crusades, the Baltic and the Catholic Frontier, 1100-1525. Macmillan, 1980. (New Studies in Medieval History)