Complex sentence: The Portuguese left in 1661 as per the marriage treaty of Charles II of England and Catherine of Braganza, daughter of King John IV of Portugal, which placed the islands in possession of the British Empire, as part of Catherine's dowry to Charles.
Split into simpler sentences: The Portuguese left in 1661 as per the marriage treaty of Charles II of England and Catherine of Braganza, daughter of King John IV of Portugal. This marriage shifted possession of the islands to the British Empire, as part of Catherine's dowry to Charles.