Complex sentence: Hesse-Kassel was then incorporated to the Kingdom of Westphalia ruled by Jérôme Bonaparte; but after the Battle of Leipzig in 1813 the French were driven out and on 21 November the Elector returned in triumph to his capital Kassel.
Split into simpler sentences: Hesse-Kassel was then incorporated to the Kingdom of Westphalia ruled by Jérôme Bonaparte. After the Battle of Leipzig in 1813 the French were driven out and on 21 November the Elector returned in triumph to his capital Kassel.