Complex sentence: Sensing that he was losing control over his men, Thorkell and his loyalists defected and he and his men entered into the service of king Ethelred the Unready, whom they fought under in 1013 against the invasion of king Sweyn Forkbeard and his son Cnut.
Split into simpler sentences: Sensing that he was losing control over his men, Thorkell and his loyalists defected. He and his men entered into the service of king Ethelred the Unready, whom they fought under in 1013 against the invasion of king Sweyn Forkbeard and his son Cnut.