On 5 June 1657, Denmark declared war on Sweden which was under heavy pressure in the Second Northern War against Poland and Russia. Although Charles X Gustav was deeply involved in the conflict with the Polish–Lithuanian Commonwealth, he chose to move the bulk of his army to Jutland and invade Denmark. The king's rapid march surprised the Danish troops, whose main body was forced to retreat to the fortress of Frederiksodde. The Swedes attacked and conquered Frederiksodde on 27 October. With Jutland secured, Charles X Gustav sought to continue his campaign towards Copenhagen on Zealand, but the Danish straits and the Danish navy obstructed him. Since the 17th century was the coldest during the Little Ice Age, with the winters in Scandinavia being exceptionally frigid, the king planned to take advantage of the weather by remaining in Jutland until sufficient ice had built up to support the weight of his troops, and then carry out a risky march across the ice.