 In 867, Charles the Bald signed the Treaty of Compiègne, by which he agreed to yield the Cotentin Peninsula (and probably the Avranchin) to the Breton king Salomon, on condition that Salomon would take an oath of fidelity and fight as an ally against the Vikings.After being defeated by the Franks (led by Robert I of France)at the Battle of Chartres in 911, the Viking leader Rollo and the Frankish King Charles the Simple signed the Treaty of Saint-Clair-sur-Epte, under which Charles gave Rouen and the area of present-day Upper Normandy to Rollo, establishing the Duchy of Normandy.In exchange, Rollo pledged vassalage to Charles and agreed to baptism.