This chapter examines how religion influenced the ways in which polities and commanders conducted war and how the influence of religion changed over the period 1350–1750. This epoch witnessed both the greatest penetration by Islamic powers into Europe, and the Protestant Reformation. However, it must not be assumed either that religion was an unimportant factor in warfare before the Ottoman expansion into central Europe of the mid-to-late fifteenth century or the conflicts between Catholic and Protestant of the mid sixteenth to mid seventeenth centuries, or that it ceased to be influential after the Peace of Westphalia (1648), traditionally regarded as the terminal date of ‘wars of religion’. In fact, religion was far from being unimportant at any point up to the mid eighteenth century – but its influence on conflict changed over the period.

Up to the mid seventeenth century, religion influenced the conduct of military operations, being one of several factors that led generals actively to avoid battles; but from the late seventeenth century on, this was no longer the case. Up to the 1520s, then, religion was important primarily as an influence on how warfare was conducted, rather than as a cause of warfare. It generated wars only on Europe's north-eastern, south-eastern, and south-western margins, and was a factor in mobilising men and resources from across Christendom for those wars.