Complex sentence: United with Russia through a personal union, with the Tsar as King of Poland, the Polish estates could elect their own parliament (the Sejm) and government, and the kingdom had its own courts, army and treasury.
Split into simpler sentences: United with Russia through a personal union with the Tsar as King of Poland, the province could elect its own parliament (the Sejm) and government. The kingdom had its own courts, army and treasury.