The treaty of Saint-Clair-sur-Epte (911) is the foundational document 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 the Simple to Rollo in return for his fealty and protection against other Viking groups that had been regularly raiding the northern coastline.^{[1][non-primary source needed]} This treaty effectively changed how mainland Europe knew to resolve ongoing raids from the Vikings.^[2]

Vikings had been raiding and plundering French lands since the age of Charlemagne. King after king among the Carolingian dynasty had tried but ultimately failed to stop these Viking incursions throughout the dynasties reign. Upon Louis the Pious death, his son Charles II the Bald took over. Charles II had immediately been thrust into a war against his brother Lothair. Lothair, who was the emperor at the time, ordered his Danish vassal to raid and plunder specific sites in Charles’ kingdom.^[3] Eventually, the Vikings became accustomed to easily obtained riches and could not be controlled resulting in them plundering whatever they wanted to, at any time they want. It got to a point where the Vikings had more of a say than the Danish King himself on where and when to raid. They hit modern day Normandy and Brittany in devastating fashion. The most important raid that occurred during his reign was when the famous Viking Ragnar attacked Paris. Charles tried to stop this plundering by force but, like his family before him, was not successful. He then opted for an approach to pay the Vikings off, but this only made the Vikings thirstier for plunder.

After Charles II the Bald died in 877, there were 6 more rulers that had to deal with the Vikings up to the Treaty of Saint Claire sur Epte, with the last being Charles the Simple. The five rulers before him; Louis the Stammerer, Louis III, Carloman II, Odo of Paris, and Charles the Fat, had not done anything more significant than their predecessors to thwart these Vikings. Throughout the times of these six kings, the counts and dukes under the kingdom in West Frankia started to fortify their own lands heavily to counter these Viking attacks. Eventually, the Vikings raids would become much harder to accomplish as these smaller feudal lords protected their land well. Nonetheless, it has still been hundreds of years of Viking invasion and permanent settlements were bound to happen. In one case, a Viking leader named Rollo, who brought thousands of Vikings to raid, plunder, and settle had enacted a permanent settlement along the Seine River in the year 900.^[3] Just two years before this in 898, a new king had been crowned called Charles the Simple. Although a bit worrisome, the permanent Viking settlement was relatively peaceful and did not pose a real threat to Charles. It was until 911 when raiding resumed from Rollo that caused the real concern of the Viking threat again, but this time things were different. Charles and Rollo would face each other in battle near Chartres resulting in a close Viking defeat.^[3] Though weakened, the Vikings were still a formidable force, which resulted in a very unusual proposal by King Charles. Charles proposed that Rollo be baptized and granted land from the Seine River to the sea, along with protecting the kingdom from further Viking raids. This treaty would go on to be known as the famous Treaty of Saint Clair Sur-Epte.