Final answer:
Silesia is a historical region in the south of Poland, extending into eastern Germany and the Czech Republic. After World War I, it became primarily part of Poland. Its complex history includes the Sudetenland crisis and subsequent German occupation of Czechoslovakia.
Step-by-step explanation:
The country with Silesia in the south, extending into eastern Germany and the Czech Republic, is Poland. Silesia is a historical region that has been part of various states over the centuries. After World War I and the Treaty of Brest-Litovsk, Silesia's boundaries were redrawn, and it became mainly part of the newly established Polish state.
The region is significant with a complex history, including the Sudetenland crisis where in 1938, the British and French allowed Germany to annex part of Czechoslovakia to protect its German-speaking population.
Later, in 1939, Germany occupied the rest of Czechoslovakia, and the Czech lands were divided between Germany and a newly-created protectorate, while Slovakia became a puppet state. Today, Silesia is mostly located within Poland's borders while also extending into the Czech Republic.