From the Christmas carol Good King Wenceslas where was good King Wenceslas the king of?

Wenceslas was the Duke of Bohemia, born around 907 in the castle of Stochov near Prague. Having been brought up with a strong Christian faith by his grandmother St Ludmila, he was a man who believed in putting his Christian faith into action. His own mother, Drahomira, was allied with an anti-Christian group that murdered Wenceslas' grandmother, allowing Drahomira to become regent in Bohemia in 920 AD when Wenceslas's own father died.

Drahomira was deposed, and Wenceslas became king of Bohemia, at the age of 18.