Complex sentence: The Zaolzie region was created in 1920, when Cieszyn Silesia was divided between Czechoslovakia and Poland with Zaolzie forming the easternmost part of the Czech portion of Cieszyn Silesia.
Split into simpler sentences: The Zaolzie region was created in 1920, when Cieszyn Silesia was divided between Czechoslovakia and Poland. Zaolzie forms the eastern part of the Czech portion of Cieszyn Silesia.