In the rapid strikes and strategic raiding that characterised medieval warfare, knowing the location and strength of the enemy forces was crucial. An ill-informed army risked blundering unexpectedly into the enemy or being taken by surprise. Basic reason would have told commanders this much, even without Vegetius's maxims to remind them: ‘In war, the one who is more vigilant in the field, the one who works harder training soldiers, will be less subject to danger’. Likewise, a commander who was able to conceal his movements and intentions was more likely to operate freely and to be able to surprise his enemy: ‘No plans are better than those you carry out while the enemy is unaware of them’. All of which was easier said than done, especially in an age before dedicated military colleges, precise maps or specialist intelligence services. Nevertheless, the very fact that medieval armies assembled and campaigned effectively indicates that they were capable of a certain level of intelligence gathering. Chronicles contain a number of hints about how this might have been achieved and more than a few stories about cunning spies.

Misdirection: Achieving the ‘Mastery of Space and Time’

Carl von Clausewtiz, Prussian general and seminal military theorist, defined military strategy as follows:

This was also true for medieval warfare. Verbruggen has demonstrated that medieval campaigns were conducted according to sound strategic principles: the maintenance of collective morale, coordinating and concentrating one's forces, attempting to attack with superior numbers and the element of surprise and, when necessary, seeking a decisive engagement with the enemy force. Surprise is particularly relevant for this study, as it usually necessary to deceive the person you wish to surprise: if somebody is aware of your intentions and actions, they cannot be surprised by what you do.