War, a powerful and enduring cultural force in the medieval West from the early Middle Ages to the fifteenth century, played a shaping role in the imaginative literature of the period. In England, warfare was a constant. The establishment of Roman Britain probably involved some degree of war against the Celts, who in turn fought along with Roman Britons against the invading Saxons, perhaps with the help of a dux bellorum in whom King Arthur finds his origins. Anglo-Saxon England was devastated by the raids of the Vikings, and finally conquered by the Normans, whose own territory of Normandy was conquered by the French king, Philip Augustus (1202-4). Conflict between France and England endured from Philip's rivalry with King John over Flanders onwards. From 1294 until 1485, from the reign of Edward I to Henry VII, England was almost constantly at war with France and France's ally, Scotland. The Hundred Years War (1337-1453) was only a continuation of age-old rivalries. Not all wars were fought against other countries: civil war ended the reigns of five medieval kings. The Wars of the Roses (1455-87) demonstrate especially well the prevalence of violence in this period: an aristocratic struggle escalated into violent factionalism, and finally into civil war, peaking in the Battle of Towton (1461), the largest battle ever fought on British soil, in which some 28,000 men died.