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.

[5] 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.

[citation needed] 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.

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.

[4] 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.

The Polish king, John II Casimir of Poland (1648–68) of the House of Vasa, eventually fled to Silesia after his armies had suffered defeats.

On 18 November 1655 the Swedes invested 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.

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.

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.

[7] In the meantime, the Russians signed a cease-fire with the Polish–Lithuanian Commonwealth (Truce of Vilna) and then pursued a campaign in Livonia and laid siege to Riga, the second largest city in the Swedish Realm.

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.

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 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, 23–24 October, whereupon he began to make preparations for conveying his troops over to Funen in transport vessels.

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.

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.

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.

On 15 January, court physician Johann Köster arrived, and in medical error mistook Charles X Gustav's pneumonia for scorbut and dyspepsia.

Before his marriage, his mistress Märta Allertz gave birth to their well-known son Gustaf Carlson (1647–1708), who became Count of Börringe and Lindholmen Castle in Scania.