and increased. By this time Charles had discovered that it was easier to defeat the Poles than to conquer Poland. His chief object, the conquest of Prussia, was still unaccomplished, and a new foe arose in the elector of Brandenburg, alarmed by the ambition of the Swedish king. Charles forced the elector, indeed, at the point of the sword to become his ally and vassal (treaty of Königsberg, Jan. 17, 1656); but the Polish national rising now imperatively demanded his presence in the south. For weeks he scoured the interminable snow-covered plains of Poland in pursuit of the Polish guerillas, penetrating as far south as Jaroslau in Galicia, by which time he had lost two-thirds of his 15,000 men with no apparent result. His retreat from Jaroslau 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, was one of his most brilliant achievements. But his necessities were overwhelming. On the 21st of June Warsaw was retaken by the Poles, and four days later Charles was obliged to purchase the assistance of Frederick William by the treaty of Marienburg. On July 18-20 the combined Swedes and Brandenburgers, 18,000 strong, after a three days’ battle, defeated John Casimir’s army of 100,000 at Warsaw and reoccupied the Polish capital; but this brilliant feat of arms was altogether useless, and when the suspicious attitude of Frederick William compelled the Swedish king at last to open negotiations with the Poles, they refused the terms offered, the war was resumed, and Charles concluded an offensive and defensive alliance with the elector of Brandenburg (treaty of Labiau, Nov. 20) whereby it was agreed that Frederick William and his heirs should henceforth possess the full sovereignty of East Prussia.
This was an essential modification of Charles’s Baltic policy; but the alliance of the elector had now become indispensable on almost any terms. So serious, indeed, were the difficulties of Charles X. in Poland that it was with extreme satisfaction that he received the tidings of the Danish declaration of war (June 1, 1657). The hostile action of Denmark enabled him honourably to emerge from the inglorious Polish imbroglio, and he was certain of the zealous support of his own people. He had learnt from Torstensson that Denmark was most vulnerable if attacked from the south, and, imitating the strategy of his master, he fell upon her with a velocity which paralysed resistance. At the end of June 1657, at the head of 8000 seasoned veterans, he broke up from Bromberg in Prussia and reached the borders of Holstein on the 18th of July. The Danish army at once dispersed and the duchy of Bremen was recovered by the Swedes, who in the early autumn swarmed over Jutland and firmly established themselves in the duchies. But the fortress of Fredriksodde (Fredericia) held Charles’s little army at bay from mid-August to mid-October, while the fleet of Denmark, after a stubborn 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 an offensive and defensive alliance was concluded between Denmark and Poland. Still more ominously, the elector of Brandenburg, perceiving Sweden to be in difficulties, joined the league against her and compelled Charles to accept the proffered mediation of Cromwell and Mazarin. The negotiations foundered, however, upon the refusal of Sweden to refer the points in dispute to a general peace-congress, and Charles was still further encouraged by the capture of Fredriksodde (Oct. 23-24), whereupon he began to make preparations for conveying his troops over to Fünen in transport vessels. But soon another and cheaper expedient presented itself. In the middle of December 1657 began the great frost which was to be so fatal to Denmark. In a few weeks the cold had grown so intense that even the freezing of an arm of the sea with so rapid a current as the Little Belt became a conceivable possibility; and henceforth meteorological observations formed an essential part of the strategy of the Swedes. On the 28th of January 1658, Charles X. arrived at Haderslev (Hadersleben) in South Jutland, when it was estimated that in a couple of days the ice of the Little Belt would be firm enough to bear even the passage of a mail-clad host. The cold during the night of the 29th of January was most severe; and early in the morning of the 30th the Swedish king gave the order to start, the horsemen dismounting where the ice was weakest, and cautiously leading their horses as far apart as possible, when they swung into their saddles again, closed their ranks and made a dash for the shore. The Danish troops lining the opposite coast were quickly overpowered, and the whole of Fünen was won with the loss of only two companies of cavalry, which disappeared under the ice while fighting with the Danish left wing. Pursuing his irresistible march, Charles X., with his eyes fixed steadily on Copenhagen, resolved to cross the frozen Great Belt also. After some hesitation, he accepted the advice of his chief engineer officer Eric Dahlberg, who acted as pioneer throughout and chose the more circuitous route from Svendborg, by the islands of Langeland, Laaland and Falster, in preference to the direct route from Nyborg to Korsör, which would have been across a broad, almost uninterrupted expanse of ice. Yet this second adventure was not embarked upon without much anxious consideration. A council of war, which met at two o’clock in the morning to consider the practicability of Dahlberg’s proposal, at once dismissed it as criminally hazardous. Even the king wavered for an instant; but, Dahlberg persisting in his opinion, Charles overruled the objections of the commanders. On the night of the 5th of 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, fearing every moment lest the rotting ice should break beneath their feet. At three o’clock in the afternoon, Dahlberg leading the way, the army reached Grimsted in Laaland without losing a man On the 8th of February Charles reached Falster. On the 11th he stood safely on the soil of Sjaelland (Zealand). Not without reason did the medal struck to commemorate “the glorious transit of the Baltic Sea” bear the haughty inscription: Natura hoc debuit uni. An exploit unique in history had been achieved.
The crushing effect of this unheard-of achievement on the Danish government found expression in the treaties of Taastrup (Feb. 18) and Roskilde (Feb. 26, 1658), whereby Denmark sacrificed nearly half her territory to save the rest (see Denmark: History). But even this was not enough for the conqueror. Military ambition and greed of conquest moved Charles X. to what, divested of all its pomp and circumstance, was an outrageous act of political brigandage. At a council held at Gottorp (July 7), Charles X. resolved to wipe from the map of Europe an inconvenient rival, and without any warning, in defiance of all international equity, let loose his veterans upon Denmark a second time. For the details of this second struggle, with the concomitant diplomatic intervention of the western powers, see Denmark: History, and Sweden: History. Only after great hesitation would Charles X. consent to reopen negotiations with Denmark direct, at the same time proposing to exercise pressure upon the enemy 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 murmured at the imposition of fresh burdens; and Charles had need of all his adroitness to persuade them that his demands were reasonable and necessary. At the very beginning of the Riksdag, in January 1660, it was noticed that the king was ill; but he spared himself as little in the council-chamber as in the battle-field, till death suddenly overtook him on the night of the 13th of February 1660, in his thirty-eighth year. The abrupt cessation of such an inexhaustible fount of enterprise and energy was a distinct loss to Sweden; and signs are not wanting that, in his latter years, Charles had begun to feel the need and value of repose. Had he lived long enough to overcome his martial ardour, and develop and organize the empire he helped to create, Sweden might perhaps have remained a great power to this day. Even so she owes her natural frontiers in the Scandinavian peninsula to Charles X.