The territory covered by the treaty is nearly the same as today’s Upper Normandy down to the Seine river. It would later extend west beyond the Seine to form the Duchy of Normandy, named for the northmen who ruled it. The treaty was entered into after the death of Alan I, King of Brittany and at the time another group of Vikings occupied Brittany. Around 937 Alan I's son, Alan II returned from England to expel those Vikings from Brittany. This happened in 939. During this period the Cotentin Peninsula was lost by Brittany and gained by Normandy. 