Complex sentence: Some historians, including W. M. Aird, have suggested that St-Calais felt that the division of the Conqueror's realm between two sons was unwise, and that reuniting the Normans and English under one king was the reason St Calais joined the rebellion.
Split into simpler sentences: Some historians, including W. M. Aird, have suggested that St-Calais felt that the division of the Conqueror's realm between two sons was unwise. Reuniting the Normans and English under one king has been suggested as the reason St Calais joined the rebellion.