The treaty of Saint-Clair-sur-Epte (911) is the foundational treaty of the Duchy of Normandy, establishing Rollo, a Norse warlord and Viking leader, as the first Duke of Normandy in exchange for his loyalty to Charles III, the king of West Francia, following the Siege of Chartres. This treaty included a land grant of farmable coastal land from Charles to Rollo in return for Rollo's fealty and help against other Viking groups that had been regularly raiding the northern coastline. This treaty effectively changed how mainland Europe resolved ongoing raids from the Vikings.