Complex sentence: It is important to note that Rollo stayed true to his word of defending the shores of the Seine river in accordance to the Treaty of Saint-Clair-sur-Epte, but in time he and his followers had very different ideas.
Split into simpler sentences: It is important to note that 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, in time he and his followers had very different ideas.