A larger-than-life character, Kristian IV (1577-1648) was on the Danish throne for 52 years, building palaces and towns, turning Copenhagen into an important European capital, trebling the size of the navy, and sending explorers to investigate the possibility of a northwest passage to Asia.
He took an avuncular interest in Norway, renaming Oslo after himself (“Kristiania”), but never managed to achieve 
friendly relations with Sweden, and the two countries inevitably entered the Thirty Years War on opposite sides.
The battling continued, but it was the Swedish King Karl X Gustav who seized an opportunity when the Sound
(Øresund) froze over during the winter of 1657. Two cavalry squadrons crossed the ice and a resounding victory after a battle on ice cost Denmark all its territory in the Swedish side of the Sound.