Complex sentence: After 911, Rollo stayed true to his word of defending the shores of the Seine river in accordance to the Treaty of Saint-Clair-sur-Epte, however he also continued to act like a Viking chief with attacks on Flanders.
Split into simpler sentences: After 911, Rollo stayed true to his word of defending the shores of the Seine river in accordance to the Treaty of Saint-Clair-sur-Epte. However, he also continued to act like a Viking chief with attacks on Flanders.