Complex sentence: After a brief period of existence in the inter-war period the Senate was again abolished (by many accounts illegally) by the authorities of the People's Republic of Poland, it was not re-established until the collapse of communism and rebirth of democracy in Poland in 1989.
Split into simpler sentences: After a brief period of existence in the inter-war period the Senate was again abolished (by many accounts illegally) by the authorities of the People's Republic of Poland. It was not re-established until the collapse of communism and rebirth of democracy in Poland in 1989.