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 2nd 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 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.

The enterprise was full of risk. The ice swayed perilously beneath the marching hosts. At places it broke. But the island shore was safely reached, the troops guarding it were beaten, and soon the whole island was in Charles's possession.

But a more daring and perilous enterprise confronted the king. There was a broader arm of the sea to cross, the Great Belt, about twelve miles wide. The ice was examined and tested by the quartermaster-general, who said that he would answer with his life for its being strong enough to bear the army.

King Charles heard this tidings with delight, clapping his hands energetically and exclaiming:

"Now, Brother Frederick, we will converse with each other in good Swedish."

Dahlberg, the quartermaster-general, testified to his confidence by riding at the head of the column over the wide field of ice, the army following in safety to the coast of Zealand. Meeting with no opposition, Charles and his army were soon near Copenhagen, whose fortifications were in bad condition, and the danger of losing his capital was so imminent that Frederick was glad to accept the severe terms of peace which Charles offered him. These included the surrender of half a dozen Danish provinces to Sweden and the independence of the Duke of Holstein-Gottorp from Danish control. Denmark had paid sorely for making a declaration of war with no preparation to carry it out.

But Charles X. was so eager for war that in the end he lost most of what he had gained. He was full of schemes of conquest in Germany, but feared that Denmark might take advantage of his absence with his army to take revenge for her losses. The fleets of Holland were threatening the coasts of the Baltic Sea, and Charles sought to make a treaty with Denmark which would close this sea to foreign ships. Denmark refused to enter such an alliance and Charles thereupon determined to make a complete conquest of that kingdom.

Breaking without warning the treaty of peace he had recently made, he suddenly landed with an army on the coast of Zealand. By this unwarranted and stealthy assault he filled the souls of the Danes with the courage of despair, changed Holland from a secret to an open enemy, and lost the most of his former gains.

The Danish people, threatened with the loss of their independence, flew to arms, determined to defend their country to the last extremity. Charles, his army being small, delayed his attack upon Copenhagen, which might easily have been taken by an immediate assault. When he appeared before it he found all its people converted into armed soldiers, while King Frederick declared that he was ready to die in his capital like a bird in its nest. Every soul in the city burned with patriotism, and nobles, burghers, and laborers alike manned the walls, while even women could be seen wielding spade and axe in the repair of the neglected defences. When the siege began the citizens made several successful sallies against their foes and hope arose in their breasts.

But their position soon grew critical, the Swedes seizing the castle of Cronberg and other points commanding the Sound and pushing forward their lines until they had possession of the outer works of the city. The great weakness of the citizens lay in the absence of provisions, which grew so scarce that they would have had to surrender from sheer stress of hunger but for the activity of their allies.

The Dutch had enlisted in their cause, and a fleet sent from Holland under Admirals Opdam and DeWitte passed Cronberg and other fortifications held by the Swedes, met the Swedish fleet under Admiral Vrangel in the Sound and fought a bloody battle for the mastery. For six hours the thunder of cannon echoed from the neighboring shores, then the Swedes were put to flight and a favoring wind bore the Dutch ships triumphantly to the beleagured city, bringing food and help to the half-starved defenders.

Their coming saved Copenhagen. Charles, baffled in his efforts, drew back, and threw up works of defence ten miles from the city. Suddenly the tide of fortune had turned and began to run strongly against him. Into Holstein pressed an invading army of Austrians, Poles, and Brandenburgers. The Swedes were forced to evacuate Jutland. The newly won provinces were ready to revolt. Part of those held in Norway were taken by the Danes, and the Swedish garrison in the island of Bornholm was annihilated by a sudden revolt of the inhabitants.

When winter came and the waters were closed by ice against invading fleets, the Swedish king determined to make a vigorous effort to take the city by assault. The attack was made on the night of February 10, 1659, Generals Stenbock and Spane leading a storming party against the fortifications. Fortunately for the people, they had information of the coming assault and were fully prepared for defence, and a desperate struggle took place at the walls and in the frozen ditches. The fire of a multitude of cannon served to light up the scene, and the attacking Swedes found themselves met with the frantic courage of men and women fighting for their homes. A shower of bullets and stones burst upon them, many women taking part, throwing burning brands, and pouring boiling tar upon their heads. In the end the Swedes were forced to draw back, leaving two thousand dead and wounded in the hands of their foes.

Relinquishing his attack upon the city, Charles now turned furiously upon the small islands of Laaland, Falster, Moen, and Langeland, which had offended him by supplying provisions for the city, and subjected them to all the horrors of invasion by troops to whom every excess of outrage was allowed. Yet new misfortunes gathered round him, the peninsula of Fyen being taken by the allies of Denmark, while the Swedish troops near Nyberg were attacked and taken prisoners, their commander alone escaping in a small boat.

The intervention offered by the neighboring powers was refused by the proud Swedish king, who, surrounded by dangers on all sides, now issued a call for a meeting of the estates of the realm at Gothenburg, while at the same time preparing to invade Norway as a part of the Danish dominions. At this interval he was suddenly taken sick and died soon after reaching Gothenburg. A treaty followed with the widowed queen, regent of Sweden, and Frederick preserved his realm, though not without loss of territory.