Complex sentence: After World War I, the administration had to be reorganised because the Posen district was split, The eastern art of the county was handed over to Poland in 1920, according to the Versailles treaty.
Split into simpler sentences: After World War I, the administration had to be reorganised because the Region of Posen was split. Thus the eastern part of the county was handed over to Poland in 1920, according to the Versailles treaty.