Sweden's armies, under the command of Lennart Torstensson, suddenly invade Denmark in 1643, without declaring war.

The ensuing conflict becomes known as the Torstenson War.

The Netherlands, wishing to end the Danish stranglehold on the Baltic, join the Swedes in their war against Denmark–Norway.

In October 1644 a combined Dutch-Swedish fleet destroys eighty percent of the Danish fleet in the Battle of Femern.

The result of this defeat proves disastrous for Denmark–Norway: in the Second treaty of Brömsebro (1645), Denmark cedes to Sweden the Norwegian provinces Jemtland, Herjedalen and Älvdalen as well as the Danish islands of Gotland and Øsel.

Halland goes to Sweden for a period of thirty years and the Netherlands are exempted from paying the Sound Duty.

Nevertheless, Danes remember Christian IV as one of the great kings of Denmark.

He has a very long reign, from 1588 to 1648, and has become known as "the architect on the Danish throne" because of the large number of building projects he undertook.

Many of the great buildings of Denmark date from his reign.

After the death of Christian IV in 1648, his son Frederick succeeds him.

Christian IV of Denmark has never lost hope of retrieving his fortunes, even at their lowest ebb, and the European situation between 1629 and 1643 has presented infinite possibilities to politicians with a taste for adventure.

Christian is no statesman, and is incapable of a consistent policy.

He has neither conciliated Sweden, henceforth his most dangerous enemy, nor has he guarded himself against her by a definite system of counter-alliances.

By mediating in favor of the emperor, after the death of Gustavus Adolphus in 1632, he had tried to minimize the influence of Sweden in Germany, and had gleaned some minor advantages, but his entire Scandinavian policy is so irritating and vexatious that Swedish statesmen make up their minds that a war with Denmark is only a question of time.

It seems to them in the spring of 1643 that the time had come.

The Swedes, thanks to their conquests in the Thirty Years' War, are now able to attack Denmark from the south as well as the east; the Dutch alliance promises to secure them at sea, and an attack upon Denmark will prevent her from utilizing the impending peace negotiations to the prejudice of Sweden.

The Swedish Privy Council in Ma decides upon war.

Sweden conquers Denmark’s German territories of Bremen and ...

...Verden in 1643, administered until now by Christian IV’s younger son, Frederick.

The Swedish Field Marshal Lennart Torstensson, advancing from Bohemia, on December 12 crosses the southern frontier of Denmark.

The entire peninsula of Jutland is in the possession of the Swedish forces by the end of January 1644.

This totally unexpected attack, conducted from first to last with consummate ability and lightning-like rapidity, has a paralyzing effect upon Denmark.

Christian IV knows his duty—fortunately for his subjects, amid almost universal helplessness and confusion—and has the courage to do it.

Christian IV, displaying something of the magnificent energy of his triumphant youth, labors night and day to levy armies and equip fleets.

Fortunately for him, the Swedish government delays hostilities in Scania until February 1644, so that the Danes are able to make adequate defensive preparations and save the important fortress of Malmö.

Lennart Torstensson, the Swedish field marshal, is unable to cross from Jutland to Funen for want of a fleet, and the Dutch auxiliary fleet which comes to his assistance is defeated between the islands of Sylt and Rømø on the west coast of Schleswig by the Danish admirals.