Prior to the Normans settling the land, the region that would become Normandy was constantly raided by the Vikings who sought out adventures and riches. The many churches and small defenses provided easy pickings for local cities like Caen, Evreux, and Rouen. Eventually, the fate of the Normans would forever change after their loss at the Battle of Chartres in 911 CE to Frankish forces led by Robert I of France. Upon the conclusion of the conflict, the Vikings and their leader, Rollo, signed the Treaty of Saint-Clair-Sur-Epte with the Frankish King Charles the Simple. According to the treaty, Charles gave Rollo the present-day region of Upper Normandy to Rollo, which included the city of Rouen. By doing this, the Duchy of Normandy was officially established with Rollo as its duke. In exchange, Rollo pledged vassalage to Charles, agreeing to defend northern Frankish lands and estuaries along the Seine from future Viking attacks, as well as being baptized in the Catholic Church.
With the treaty signed and the Vikings settling the land, the legend of the Northmen, or "Normans," had officially begun. As the Vikings became accustomed to living in their new lands, the influence of the Catholic faith and French culture began to affect the Vikings who settled on the land. This blend of Franco-Viking cultures was distinct from the rest of France and became just one solidified aspect of everyday Norman life. Another of these was the constant wars for land in the region. By the 11th century, the modern-day borders of Normandy were established in the Duchy, but there were still land-hungry knights and petty rulers who sought out more land to call their own. In 1066 CE, many Normans answered William the Bastard's call to invade England, where he was named King of England and earned the moniker "The Conqueror." However, there were other Normans who left decades earlier, seeking opportunities in the politically complex and fertile landscape of the Mediterranean Sea.