The Second Northern War (1655–60), also First or Little Northern War) was fought between Sweden and its adversaries the Polish–Lithuanian Commonwealth (1655–60), the Tsardom of Russia (1656–58), Brandenburg-Prussia (1657–60), the Habsburg Monarchy (1657–60) and Denmark–Norway (1657–58 and 1658–60). The Dutch Republic waged an informal trade war against Sweden and seized the colony of New Sweden in 1655, but was not a recognized part of the Polish–Danish alliance.

In 1655, Charles X Gustav of Sweden invaded and occupied western Poland–Lithuania, the eastern half of which was already occupied by Russia. The rapid Swedish advance became known in Poland as the Swedish Deluge. The Grand Duchy of Lithuania became a Swedish fief, the Polish–Lithuanian regular armies surrendered and the Polish king John II Casimir Vasa fled to the Habsburgs. Frederick William, Elector of Brandenburg and Duke of Prussia initially supported the estates in Royal Prussia, but allied with Sweden in return for receiving the Duchy of Prussia as a Swedish fief. Exploiting the hurt religious feelings of the Roman Catholic population under Protestant occupation and organizing Polish–Lithuanian military leaders in the Tyszowce Confederation, John II Casimir Vasa managed to regain ground in 1656. Russia took advantage of the Swedish setback, declared war on Sweden and pushed into Lithuania and Swedish Livonia.

Charles X Gustav then granted Frederick William full sovereignty in the Duchy of Prussia in return for military aid, and in the Treaty of Radnot allied himself with the Transylvanian George II Rákóczi who invaded Poland–Lithuania from the southeast. John II Vasa found an ally in Leopold I of Habsburg, whose armies crossed into Poland–Lithuania from the southwest. This triggered Frederick III of Denmark's invasion of the Swedish mainland in early 1657, in an attempt to settle old scores from the Torstenson War while Sweden was busy elsewhere. Brandenburg left the alliance with Sweden when granted full sovereignty in the Duchy of Prussia by the Polish king in the treaties of Wehlau and Bromberg.

Frederick III's war on Sweden gave Charles X Gustav a reason to abandon the Polish–Lithuanian deadlock and fight Denmark instead. After marching his army to the west and making a dangerous crossing of the frozen straits in the winter of 1657/58, he surprised the unprepared Frederick III on the Danish isles and forced him into surrender. In the Treaty of Roskilde, Denmark had to abandon all Danish provinces in what is now Southern Sweden. The anti-Swedish allies meanwhile neutralized the Transylvanian army and Polish forces ravaged Swedish Pomerania.

In 1658 Charles X Gustav decided that instead of returning to the remaining Swedish strongholds in Poland–Lithuania, he would rather attack Denmark again. This time, Denmark withstood the attack and the anti-Swedish allies pursued Charles X Gustav to Jutland and Swedish Pomerania. Throughout 1659, Sweden was defending her strongholds in Denmark and on the southern Baltic shore, while little was gained by the allies and a peace was negotiated. When Charles X Gustav died in February 1660, his successor settled for the Treaty of Oliva with Poland–Lithuania, the Habsburgs and Brandenburg in April and the Treaty of Copenhagen with Denmark in May. Sweden was to keep most of her gains from Roskilde, the Duchy of Prussia became a sovereign state, and otherwise, the parties largely returned to the status quo ante bellum. Sweden had already concluded a truce with Russia in 1658, which gave way to a final settlement in the Treaty of Cardis in 1661.