Complex sentence: In 1806, William I was dispossessed by Napoleon Bonaparte for his support of the Kingdom of Prussia, and Kassel became the capital of a new Kingdom of Westphalia with Napoleon's brother Jérôme Bonaparte as king.
Split into simpler sentences: In 1806, William I was dispossessed by Napoleon Bonaparte for his support of the Kingdom of Prussia. Kassel was designated as the capital of a new Kingdom of Westphalia, where Napoleon appointed his brother Jérôme Bonaparte as king.