John II. Casimir of Poland, the younger son of Sigismund, like Vladislav, styled himself king of Sweden and had claims to Livonia. For this he should be punished. It was not the original intention of Charles X. to make himself king of Poland, but he was probably the first who ever devised a division of that unhappy country. The success of Charles X. was without a parallel. The strong fortresses were captured, the armies surrendered and registered in Swedish service. After two months Charles X. entered the old capital of Cracow, John Casimir fled from his country, and, carried away by the frenzy of success, Charles Gustavus had himself crowned king of Poland. West Prussia was captured, and the elector of Brandenburg, who held East Prussia in fief, and the duke of Courland were forced to become the vassals of Sweden, in 1656.
But Charles X. had roused an enemy that few invaders, however great, have been able to successfully encounter, the spirit of patriotism. The Poles, enticed to revolt by the Catholic clergy, found a leader in the noble Czarniecki, who commenced a war of liberation on the Swedish usurper. King John Casimir returned, and armies were gathered. Charles Gustavus was yet to do wonders of strategy, which aroused the amazement and fear of all Europe, but he was glad, when finding a good excuse, to extract himself from the affairs of Poland. In 1656 he defeated Czarniecki at Golumbo, undertook the adventurous crossing of the river of San, and captured, and recaptured, the capital of Warsaw. The “three days’ battle of Warsaw” (18th-20th of July, 1656) is one of the most famous in modern warfare, by which the reputation of Charles X., as one of the greatest warriors of his time, was firmly established. Charles X. had joined forces with the “great elector” of Brandenburg, who up to the last moment was unwilling to risk a battle of 22,000 men against an enemy twice as strong. Charles Gustavus was unyielding and turned it into a great victory. But his position became precarious, Russia, Germany, Holland and Denmark being hostile, joined by Brandenburg, the ambitious “great elector” not being satisfied with the Swedish supremacy in East Prussia. To save himself from the dilemma with untarnished glory, Charles X. decided to fight Denmark, which country had declared war without suspecting the possibility of an attack.