Charles burned with desire to gain fame in war, taking for pretext that the king of Poland, by his repeated claims to the Swedish throne, made peace treacherous and impossible. But such was the condition of affairs that something must be done to quiet the malcontent people, restore peace between the quarrelling classes, and reimburse the empty state treasury. At a Riksdag in Stockholm, in 1655, a restitution was proposed by the king and agreed to, according to which all estates which in earlier times had been rendering dues to the direct support of the court, army, fleet, or administration, should be confiscated to the crown; also one-fourth of the estates given away since the death of Gustavus Adolphus, and all estates fraudulently obtained. A committee to enforce the restitution was appointed, to be presided over by the able Herman Fleming. The restitution, far from radical in itself, was not completely carried through, thanks to the opposing nobles. But it proved effective for the moment, the king securing the goodwill of the people, temporary quiet and means to carry on the proposed war, to commence which Charles Gustavus received the somewhat reluctant consent of the faithful people whose financial state was a most despairing one. Charles X. thought in new conquests to find means to better their condition. Shortly after his coronation he married Hedvig Eleonore of Holstein-Gottorp.
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.
Lennart Torstensson, his master of strategy, had shown Charles X. how Denmark was to be attacked. With an army of only 8,000, but consisting of the choicest and most victorious troops in all Europe, Charles X. hastened in rapid marches through Pomerania and Mecklenburg, recaptured Bremen, and invaded through friendly Holstein all of Schleswig and Jutland, defeating the larger but inexperienced Danish army and capturing the strong fortress of Fredericia.
Yet the new position was as precarious as the one in Poland, and Charles had to use all the skill of his diplomacy to save his little army from an assault by inimical Europe. France and England seemed unwilling to render him effective help. But when the elector of Brandenburg, who had taken upon himself the leadership of Sweden’s enemies, turned to the emperor, emphasizing the necessity of crushing the Swedish power in one blow, he received the following surprising answer: “The king of Hungary has no reason to be the enemy of the king of Sweden.” Charles had reached a secret understanding with Austria. By this move he gained time. Through what seemed almost a miracle, he was not only to save his army but lead it on to victory after a strategic deed, in originality and daring unique in the history of the world.
The year of 1658 commenced with severe frosts. Charles X. conceived the daring plan of attacking the Danish isles by leading his army over the frozen sounds. He concluded to cross the sound of Lille Belt, opposite the islet of Brandsœ. His quartermaster-general, Eric Dahlberg, an engineer of great genius, ascertained that the ice was safe. One frosty winter morning, the 30th of January, the Swedish army, reinforced to 9,000 men, marched down on the ice, safely reaching Brandsœ at sunrise. A Danish army, arranged in order of battle in the island of Funen, was defeated. While crossing over to Funen, the ice cracked under two squadrons of cavalry, those who followed not daring to proceed. The king himself hurried past the dangerous place, pointing out a safe course, and the troops followed him.
The most dangerous part remained to cross, the much wider sound of Store Belt, in order to reach Seeland. Charles first thought of taking the direct route of two miles, but commissioned Dahlberg to explore the condition of the ice across to the smaller islands to the south. Dahlberg did so, and said he would wager his head for its perfectly safe condition. In enthusiasm, Charles clapped his hands exclaiming: “Now, brother Frederic, we will converse in good Swedish!” In the night between the 5th and 6th of February, the Swedish army marched from Svendborg in Funen over the ice to Langeland. “It was terrible,” wrote an eye-witness, “to march through the night over this frozen sea, where the horses’ hoofs had thawed down the snow on the ice, which was below two feet of water, and where we, in every moment, were in fear of striking the open sea.” At dawn the army landed in Langeland. During the rest for breakfast, frozen beer was chopped and distributed in pieces to the soldiers. The march continued over the still wider sound to Laaland, Eric Dahlberg in front, directing the march. Reaching Grimsted in Laaland at three o’clock in the afternoon, and proceeding to Nakskov, Charles Gustavus was met, at midnight, by the burgomaster and council of said town, who surrendered its keys. The 9th of February, the army stood in Falster, and a few days later was collected at the captured castle of Vordingborg in Seeland. Peace was hastily offered and agreed to on the 17th, and the treaty of peace signed the 28th of February, 1658, at Rœskilde. The conditions were severe, Denmark ceding the provinces of Scania, Halland, Bleking and Bohuslæn, the whole district or diocese of Drontheim in Norway, and the island of Bornholm, and agreeing to hold the Baltic closed to hostile fleets with the help of Sweden. The last clause was a piece of a Scandinavian policy devised by the Swedish king.
Charles X. now prepared to meet Brandenburg and Austria, once more siding against Sweden. It was necessary to keep Holland out from the Baltic, and when Charles X. found Denmark unwilling to keep the conditions of the recent treaty on that point, the war-lord became wrathful, dooming obnoxious Denmark to lose her very existence. But the spirit of patriotism, which so often had saved Sweden in instances of extreme danger, now sided with Denmark, as it had already sided with Poland. King Frederic declared he would die like a bird in its own nest, and roused the patriotism of the population of Copenhagen, which, badly defended, was hurriedly fortified at the news of an intended attack. Contrary to the advice of Eric Dahlberg, Charles X. made no instantaneous attack, but commenced a siege, although he did not bring with him the necessary means. The castle of Kronborg by Elsinore was captured and its cannon used against Copenhagen. The greatest enthusiasm prevailed in the Danish capital; the king slept in a tent by the fortifications, and especially the students and Norwegian sailors distinguished themselves by their valor and patriotism. Charles X. found it impossible to take Copenhagen and retired to some distance from the capital. What caused Charles to retire was the arrival of a Dutch fleet. It had been met by the Swedish fleet, under command of Charles Gustavus Wrangel, the hero of Fredericia and Kronborg. After six hours of hard fighting the Dutch forced the entrance to the Sound. Before leaving, the Swedish king resolved to make a desperate effort to capture Copenhagen, defended by 13,000 troops and by a patriotic population, with his 8,000 Swedes. The attack was made in the night of February 11, 1659, but the city, forewarned by traitors, tendered the Swedes a warm reception, consisting of artillery fire, stones, and scalding hot water. The Swedes lost 600 men and suffered their first and only defeat under the command of Charles X.
The situation was grave. The Swedish army in Jutland was forced to retire; the troops of 5,000 men in Funen were defeated and made prisoners; a revolt took place in Bornholm, and the Danes recaptured the district of Drontheim. The powers united in their efforts to force Sweden and Denmark to a treaty of peace on the basis of the Rœskilde stipulations. Charles still held his head high, declaring that he would crush the fleets of the allies if they tried to interfere in the affairs of the North, striking a sharply discordant note in the concert of the powers. To Denmark he was willing to cede the district of Drontheim, but prepared to occupy that of Akkershus instead, when he was taken ill at the convening Riksdag at Gothenburg, dying February 11, 1660, in the palace erected by his friend Lennart Torstensson.