Chroniclers appear to have assumed that their audience would know what was and was not licit behaviour in war. Beyond the legal texts that attempted to establish a theological framework for warfare, discussions about the legitimacy of deception usually occur when an author wishes to justify or defend their subject's behaviour or when discussing the cultural or religious ‘other’: those groups that were perceived to fight and behave contrary to Western European norms and were often labelled as habitually treacherous. Studying what Western chroniclers considered abnormal in ‘the other’ is another way of trying to understand what they believed to be acceptable behaviour.

Jus in Bello: Military Deceptions in Theology and Canon Law

Philosophical, ethical and theological anxieties about warfare are as old as war itself. When is it morally right to go to war? Who has the right to start or end a war? How should people act in war? Is there any way to limit its horrors? According to James Turner Johnson, in classic Western philosophy the definition of a ‘just war’ is typically divided into two parts. First, jus ad bellum: a just war must be declared by the proper authorities and for legitimate reasons. Second, jus in bello: a just war must be conducted in the right way, for example by refraining from doing harm to non-combatants or by showing clemency to enemy prisoners. Until the fourteenth century, medieval theologians and canonists had remarkably little to say about jus in bello and practically nothing about the legitimacy of particular tactics. Their primary concern was to demonstrate that it was legitimate for Christians to wage war and to determine who had the authority to declare war. This is only to be expected. The clerics and lawyers who wrote these texts had little first-hand experience of war and were not overly concerned with the niceties of military tactics.

The basis for all theological and canonical writing on stratagems in the Middle Ages was Augustine's Quaestionum in Heptateuchum libri vii, specifically his reflections on Joshua 7. In this passage, God commands Joshua and the Israelites to lay an ambush for the Canaanite citizens of Ai (as discussed in Chapter 3).