Complex sentence: In 1141, however, Stephen's foes, the Empress Matilda and her partisans, gained the upper hand and Harvey was defeated in battle and forced to forfeit his English lands and return to Brittany, almost certainly taking his Blesevin wife with him.
Split into simpler sentences: In 1141, Stephen's foes, the Empress Matilda and her partisans, gained the upper hand and Harvey was defeated in battle. He was forced to forfeit his English lands and return to Brittany, almost certainly taking his Blesevin wife with him.