Joseph R. Strager, ‘Feudalism in Western Europe’, in Rushton Coulborn, ed., Feudalism in History (Hamden, Conn., 1965), p. 21; Michael Gaddis, There is No Crime for Those Who Have Christ: Religious Violence in the Christian Roman Empire (Berkeley, Los Angeles and London, 2005), pp. 334–35; John Keegan, A History of Warfare (London, 1993), pp. 283, 289