Charles X Gustav, also Carl Gustav (Swedish: Karl X Gustav; 8 November 1622 – 13 February 1660), was King of Sweden from 1654 until his death. He was the son of John Casimir, Count Palatine of Zweibrücken-Kleeburg and Catherine of Sweden. After his father's death he also succeeded him as Pfalzgraf. He was married to Hedwig Eleonora of Holstein-Gottorp, who bore his son and successor, Charles XI. Charles X Gustav was the second Wittelsbach king of Sweden after the childless king Christopher of Bavaria. He led Sweden during the Second Northern War, enlarging the Swedish Empire. By his predecessor Christina, he was considered de facto Duke of Eyland (Öland) before ascending to the Swedish throne. His numbering as Charles X derives from a 16th-century invention. The Swedish king Charles IX (1604–1611) chose his numeral after studying a fictitious history of Sweden. This king was the fourth actual King Charles. In his early childhood raised in the Swedish court alongside his cousin Queen Christina he received an excellent civil education. Later Charles X learned the art of war under Lennart Torstenson. From 1646 to 1648 he frequented the Swedish court, supposedly as a prospective husband of his cousin the queen regnant, Christina of Sweden, but her insurmountable objection to wedlock put an end to these anticipations, and to compensate her cousin for a broken half-promise she declared him her successor in 1649, despite the opposition of the Privy Council headed by Axel Oxenstierna. In 1648 he gained the appointment of commander of the Swedish forces in Germany. The conclusion of the treaties of Westphalia in October 1648 prevented him from winning the military laurels he is said to have desired, but as the Swedish plenipotentiary at the executive congress of Nuremberg, he had an opportunity to learn diplomacy, a science he is described as having quickly mastered. As the recognized heir to the throne, his position on his return to Sweden was dangerous because of the growing discontent with the queen. He therefore withdrew to the isle of Öland until the abdication of Christina on 5 June 1654 called him to the throne. Charles Gustav was crowned on 7 June 1654, the day after his cousin Christina abdicated. The beginning of Charles X's reign concentrated on the healing of domestic discords and on the rallying of all the forces of the nation round his standard for a new policy of conquest. On the recommendation of his predecessor, he contracted a political marriage on 24 October 1654 with Hedwig Eleonora, the daughter of Frederick III, Duke of Holstein-Gottorp. He was hoping to secure a future ally against Denmark. The Riksdag which assembled at Stockholm in March 1655, duly considered the two great pressing national questions: war, and the restitution of the alienated crown lands.

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