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 Catharina 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 (1441 – 1448) and he was the first king of the Swedish Caroline era, which had its peak during the end of the reign of his son, Charles XI. 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, but has never been called Charles IV.

In his early childhood raised in the Swedish court alongside Queen Christina he received an excellent civil education. Later Charles X learned the art of war under Lennart Torstenson, being present at the second Battle of Breitenfeld (1642) and at Jankowitz (1645). From 1646 to 1648 he frequented the Swedish court, supposedly as a prospective husband of his cousin the queen regnant, Christina of Sweden (1626 – 89, reigned 1632 – 54), 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.

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. He contracted a political marriage on 24 October 1654 with Hedwig Eleonora, the daughter of Frederick III, Duke of Holstein-Gottorp, by way of securing 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. Over three days a secret committee presided over by the King decided the war question: Charles X easily persuaded the delegates that a war against Poland appeared necessary and might prove very advantageous; but the consideration of the question of the subsidies due to the crown for military purposes was postponed to the following Riksdag. In 1659 he proclaimed severe punishment for anyone hunting in the royal game reserve in Ottenby, Öland, Sweden, where he had built a long dry-stone wall separating the southern tip of the island.

On 10 July 1655, Charles X left Sweden to engage in a war against the Polish-Lithuanian Commonwealth, in what became the Second (or Little) Northern War (1655 - 1660). By the time war was declared he had at his disposal 50,000 men and 50 warships. Hostilities had already begun with the occupation of Dünaburg in Polish Livonia by the Swedes on 1 July 1655, and the Polish army encamped among the marshes of the Netze concluded a convention on 25 July, whereby the palatinates of Poznań and Kalisz placed themselves under the protection of the Swedish King. Thereupon the Swedes entered Warsaw without opposition and occupied the whole of Greater Poland. The Polish king, John II Casimir of Poland (1648 – 68) of the House of Vasa, fled to Silesia.

Meanwhile Charles X pressed on towards Kraków, which the Swedes captured after a two months' siege. The fall of Kraków followed, but before the end of the year a reaction began in Poland itself. On 18 November 1655 the Swedes attacked the fortress - monastery of Częstochowa, but the Poles defended it and after a seventy days’ siege the Swedish besiegers had to retire with great loss. This success elicited popular enthusiasm in Poland and gave rise to a nationalistic and religious rhetoric concerning the war and Charles X. He was depicted as tactless and his mercenaries barbaric. His refusal to legalize his position by summoning the Polish diet and his negotiations for the partition of the very state he affected to befriend, awoke a nationalistic spirit in the country.

In the beginning of 1656 King John II Casimir returned from exile and the reorganised Polish army, increased in numbers. By this time Charles had discovered that he could more readily defeat the Poles than conquer Poland. What is described as his chief object, the conquest of Prussia, remained unaccomplished, and a new Swedish adversary arose in the elector of Brandenburg, Frederick William I, alarmed by the ambition of the Swedish king. Charles forced the elector, albeit at the point of the sword, to become his ally and vassal (Treaty of Königsberg, 17 January 1656); but the Polish national rising now imperatively demanded his presence in the south. For weeks he engaged in the pursuit of Polish guerrillas in the snow covered plains of Poland, penetrating as far south as Jarosław in Galicia, by which time he had lost two-thirds of his 15,000 men with no apparent result. In the meantime, the Russians pursued a campaign in Livonia and laid siege to Riga, the second largest city in the Swedish Realm.

Charles's retreat from Jarosław to Warsaw, with the fragments of his host - amidst three converging armies, in a marshy forest region intersected in every direction by well guarded rivers - is considered one of his most brilliant achievements. But on 21 June 1656 the Poles retook Warsaw, and four days later Charles was obliged to purchase the assistance of Frederick William I, by the treaty of Marienburg (23 June 1656). On 28 July - 30 the combined Swedes and Brandenburgers, 18,000 strong, after a three days’ battle, defeated John II's army of 40,000 at Warsaw and reoccupied the Polish capital. However, this feat of arms did not have the desired result for Charles, and when Frederick William compelled the Swedish king to open negotiations with the Poles, they refused the terms offered, the war resumed, and Charles concluded an offensive and defensive alliance with the elector of Brandenburg (Treaty of Labiau, 20 November 1656) which stipulated that Frederick William and his heirs should henceforth possess the full sovereignty of East Prussia.

Labiau involved an essential modification of Charles's Baltic policy; but the alliance with the elector of Brandenburg had now become indispensable for him on almost any terms. The difficulties of Charles X in Poland are believed to have caused him to receive the tidings of the Danish declaration of war on 1 June 1657 with extreme satisfaction. He had learnt from Torstensson that Denmark was most vulnerable if attacked from the south, and he attacked Denmark with a velocity which paralysed resistance. At the end of June 1657, at the head of 8,000 seasoned veterans, he broke up from Bromberg (Bydgoszcz) south of Pomerania and reached the borders of Holstein on 18 July. The Danish army dispersed and the Swedes recovered the duchy of Bremen. In the early autumn Charles's troops swarmed over Jutland and firmly established themselves in the duchies. But the fortress of Fredriksodde (Fredericia) held Charles's smaller army at bay from mid August to mid October, while the fleet of Denmark, after two days’ battle, compelled the Swedish fleet to abandon its projected attack on the Danish islands. The position of the Swedish king had now become critical. In July Denmark and Poland - Lithuania concluded an offensive and defensive alliance. Still more ominously for the Swedes, the elector of Brandenburg, perceiving Sweden's difficulties, joined the league against Sweden and compelled Charles to accept the proffered mediation of Oliver Cromwell, Coenraad van Beuningen and Cardinal Mazarin. The negotiations foundered, however, upon the refusal of Sweden to refer the points in dispute to a general peace congress, and Charles received encouragement from the capture of Fredriksodde, 23 October - 24, whereupon he began to make preparations for conveying his troops over to Funen in transport vessels. But soon another and cheaper expedient presented itself. In the middle of December 1657 began the great frost, which would prove so fatal to Denmark. In a few weeks the cold had grown so intense that the freezing of an arm of the sea with so rapid a current as the Small Belt became a conceivable possibility; and henceforth meteorological observations formed an essential part of the strategy of the Swedes.

On 28 January 1658, Charles X arrived at Haderslev in South Jutland. His meteorologists estimated that in a couple of days the ice of the Little Belt would become firm enough to bear even the passage of a mail-clad host. The cold during the night of 29 January became most severe; and early in the morning of the 30th the Swedish king gave the order to start, the horsemen dismounting on the weaker spots of ice and cautiously leading their horses as far apart as possible, until they swung into their saddles again, closed their ranks and made a dash for the shore. Swedish arms quickly overpowered the Danish troops lining the opposite coast and won the whole of Funen with the loss of only two companies of cavalry, which disappeared under the ice while fighting with the Danish left wing. Pursuing his march, Charles X, with his eyes fixed steadily on Copenhagen, resolved to cross the frozen Great Belt also. However, he accepted the advice of his chief engineer officer Erik Dahlberg, who acted as pioneer throughout and chose the more circuitous route from Svendborg, by the islands of Langeland, Lolland and Falster, in preference to the direct route from Nyborg to Korsør, which would have had to cross a broad, almost uninterrupted expanse of ice. A council of war, which met at two o’clock in the morning to consider the practicability of Dahlberg's proposal, dismissed it as hazardous. Even the king wavered; but when Dahlberg persisted in his opinion, Charles overruled the objections of the commanders. On the night of 5 February the transit began, the cavalry leading the way through the snow covered ice, which quickly thawed beneath the horses’ hoofs so that the infantry which followed after had to wade through half an ell of sludge, facing the risk that the ice would break beneath their feet. At three o’clock in the afternoon, with Dahlberg leading the way, the army reached Grimsted in Lolland without losing a man; on 8 February, Charles reached Falster. On 11 February he stood safely on the soil of Zealand. A Swedish medal struck to commemorate the transit of the Baltic Sea bear the inscription: Natura hoc debuit uni. Sweden had achieved a rare war exploit, in Sweden considered to be matched only by the crusade of the Livonian Order led by William of Modena to conquer Saaremaa (Osel) in January 1227 and afterwards when two Russian armies crossed the frozen Gulf of Bothnia from Finland to mainland Sweden in March during the Finnish War. It is believed that the effect of this achievement on the Danish government found expression in the Treaty of Taastrup on 18 February, and in the Treaty of Roskilde (26 February 1658), whereby Denmark sacrificed a great part of her territory to save the rest. However, Charles X continued the war efforts against Denmark after a council held at Gottorp on 7 July, even though he was in defiance of international equity. Without warning, Denmark was attacked a second time.

On 17 July he again landed on Zealand and besieged Copenhagen with its king Frederick III of Denmark, but Copenhagen repelled a major assault and managed to hold out long enough for the Dutch fleet under Lieutenant Admiral Jacob van Wassenaer Obdam to relieve the city, defeating the Swedish fleet in the Battle of the Sound on 29 October 1658. The Dutch liberated the Danish Isles in 1659. As Baltic trade was vital to the Dutch economy they made clear to Charles they wouldn't allow Sweden to control the Sound.
Charles X consented to reopen negotiations with Denmark, at the same time proposing to exercise pressure upon his rival by a simultaneous winter campaign in Norway. Such an enterprise necessitated fresh subsidies from his already impoverished people, and obliged him in December 1659 to cross over to Sweden to meet the estates, whom he had summoned to Gothenburg. The lower estates protested the imposition of fresh burdens, but were persuaded by Charles.

Soon after the estates opened on 4 January 1660, Charles X Gustav fell ill with symptoms of a cold. Ignoring his illness, he repeatedly went to inspect the Swedish forces near Gothenburg, and soon broke down with chills, headaches and dyspnoea. On 15 January, court physisian Johann Köster arrived, and in medical error mistook Charles X Gustav's pneumonia for scorbut and dyspepsia. Köster started a "cure" including the application of multiple enemata, laxatives, bloodletting and sneezing powder. While after three weeks the fever eventually was down and the coughing was better, the pneumonia had persisted and evolved into a sepsis by 8 February. On 12 February, Charles X Gustav signed his testament: His son, Charles XI of Sweden, was still a minor, and Charles X Gustav appointed a minor regency consisting of six relatives and close friends. Charles X Gustav died the next day at the age of 37.