A worthy successor of his great uncle, Charles showed his warlike ability by a rapid march into Poland and the overthrow of its army by a three days' battle at Warsaw. But his progress was checked by a new and dark cloud which appeared upon the sky. Suddenly and unexpectedly, on the 2d of May, 1657, Denmark declared war against Sweden, and at the same time an Austrian army invaded Poland with the purpose of aiding that kingdom and destroying the Swedish army. This double attack left Charles in a quandary. An able and experienced soldier, who had learned the trade of war in Germany during Queen Christina's reign, he was well fitted to deal with one foe, but could not readily cope with two widely separated ones. He therefore determined to abandon Poland, though leaving garrisons in its more important cities, and devote his attention to Denmark. This Danish war had much in it of interest, and showed that the new Swedish king had been taught in the best school of the military art. Frederick III. of Denmark had declared war without making preparations for it, fancying that Charles would be forced to remain with his army in Poland and that he would have abundant time to act. He quickly learned his mistake. With an army of eight thousand well-trained veterans Charles marched at all speed from Poland, and a few months after war was declared stood with his compact little army on Denmark's shores. Taken by surprise, the Danish general, Bilbe, retreated hastily northward and the whole peninsula of Jutland was quickly overrun by the Swedes. Bilbe had much the larger army, but they were mainly raw recruits, and he dared not face the veterans of the Thirty Years' War. The Danes had projected an invasion of Sweden, for which they had been deliberately preparing, and were overwhelmed to find their army in retreat and a force of six thousand men closely besieged in the Fredericia fortress. A night attack by General Vrangel won this stronghold for the Swedes, with its garrison and a large amount of arms and provisions. So far the movement of Charles had been brilliantly successful, but his position was very dangerous. Enemies were advancing on him from various sides, a Polish army having invaded Pomerania, an Austrian army having advanced into Prussia, while the elector of Brandenburg had joined his enemies. His ally, England, had promised to aid him with a fleet, but it failed to appear, and the situation was growing daily more critical. From his awkward position he was rescued by a combination of daring and the favoring influences of nature. The winter of 1658 proved extraordinarily cold. Never within the memory of man had such bitter weather been known. The sea that flowed between the Danish islands was tightly frozen, a natural bridge of ice connecting them with one another and the mainland. With bold resolution King Charles determined to cross to the island of Fyen.