West Bohemia was a country that was formed after the Czechoslovakia was annexed to the USSR as Czech Soviet Socialist Republic and Slovak Soviet Socialist Republic.
It happened because this part of Czechoslovak lands was liberated by the American Army, unlike the the rest, which was liberated by the Red Army of Soviet Union.
The capital city became Pilsen because it is the largest and culturaly most developed in the country.
West Bohemia was a democratic, landlocked country in central Europe during the cold war. It shared its northern and eastern border with Soviet Union, southern border with the Austrian Federation and the western border with West German Federal Republic.