Control over Rennes, Nantes and the Pays de Retz was secured when the Frankish army was defeated once again in 851 at the Battle of Jengland by the Bretons under Erispoe; consequently Charles the Bald recognised the independence of Brittany and determined the borders that defined the historic duchy and later province. Under Erispoe's successor Salomon, Hastein's Vikings and the Bretons united as one in 865 to defeat a Frankish army at the Battle of Brissarthe, near modern-day Le Mans. Two Frankish leaders, Robert the Strong and Ranulf, were killed by the Vikings. The Franks were forced to confirm Brittany's independence from the Frankish kingdoms and expand Salomon's territory. The Vikings tactically helped their Breton allies by making devastating pillaging raids on the Frankish kingdoms. This unfortunately became a double edged sword over the next few decades as the Vikings turned on the Bretons and pillaged Brittany eventually occupying it. This situation was only overturned with the return of exiled Bretons and an alliance with the Franks. From this point Brittany became a Duchy with various levels of fealty to West Francia and eventually France.