In 1655, Karl X Gustav of Sweden invaded and occupied western Poland-Lithuania, the eastern half of which was already occupied by Russia. The rapid Swedish advance became known in Poland as Swedish Deluge. The Grand Duchy of Lithuania became a Swedish fief, the Polish-Lithuanian regular armies surrendered and the Polish king John II Casimir Vasa fled to the Habsburgs. Frederick William, Elector of Brandenburg and Duke of Prussia initially supported the estates in Royal Prussia, but allied with Sweden in return for receiving the Duchy of Prussia as a Swedish fief. Exploiting the hurt religious feelings of the Roman Catholic population under Protestant occupation and organizing Polish-Lithuanian military leaders in the Tyszowce Confederation, John II Casimir Vasa managed to regain ground in 1656. Russia took advantage of the Swedish setback, declared war on Sweden and pushed into Lithuania and Swedish Livonia.