East Poland and West Poland were once united as a monarchy ruled by Po (Who rules West Poland), but then during the years of 5012 - 5015, it got invaded by Megatron (Who rules East Poland), who then invaded the Eastern portion, containing Pokyo (Later named Warsaw).
That event during 5012 - 5015 was a war that was then named the Polish Split (Also known as the Poland War).
The end result was West Poland and East Poland both being taken over by Grand Dad and united into Grandiose Poland. 1000 years later in 6012, Weegee took over South Poland and Grandiose Poland became North Poland.
The capital of East Poland is Warsaw (Formerly Pokyo), which was the capital of Poland when it was united. For West Poland, while still having the fascist government (except becoming a republic), its capital became Wroclaw.
East Poland, despite having Warsaw/Pokyo, is a communist socialist "republic", and those who crossed the border would get killed, especially if they're from the United States of America.