The first Swedish ruler of the Palatinate dynasty, he became king when his
cousin, Queen Christina, abdicated in his favor. Charles succeeded to the
throne of an almost bankrupt kingdom; nevertheless, he hoped to increase
the territory of the country at the expense of nations weakened by the
Thirty Years' War. In 1655, on the pretext of preventing his cousin, King
John II Casimir of Poland, from seizing the Swedish crown, Charles invaded
Poland. He formed an alliance with Frederick William, elector of
Brandenburg, and was victorious at Warsaw. Soon thereafter Russia,
Denmark, and the Holy Roman Empire all declared war on Sweden, and
Frederick William switched sides when the Poles agreed to recognize his
territorial claims. Charles retired from Poland, but in 1658, leading his
army across the frozen sea, he forced the Danes to capitulate. By the
Treaty of Roskilde, King Frederick II of Denmark ceded all his lands in
southern Sweden to Charles. Hoping for a total conquest of Denmark,
Charles attacked again later in the year but was rebuffed by a stout
Danish defense of Copenhagen. Charles died soon thereafter, and he was
succeeded by his son Charles XI.