The Northern War of 1655–1660 was fought between the Swedish Empire and the Polish–Lithuanian Commonwealth, with participation at different times by Russia, Brandenburg-Prussia, the Habsburg monarchy, and Denmark–Norway. It ended with the treaties of Copenhagen and Oliva in 1660.
In 1655, Charles X took advantage of the Russo-Polish War (1654–67) to over-run western Poland. The Grand Duchy of Lithuania was annexed by Sweden, and John II Casimir Vasa took refuge in Vienna. He managed to regain parts of his kingdom in 1656, and the conflict widened when Russia declared war on Sweden, supported by Emperor Leopold and Frederick III of Denmark.
Previously allied with Sweden, Brandenburg switched sides in 1657 when Casimir granted Frederick William I sovereignty over the Duchy of Prussia. That winter, Charles X invaded Denmark and forced Frederick to withdraw from Southern Sweden. However, a second Swedish offensive failed, and by 1659 the war had become one of attrition, with neither side able to gain a decisive advantage.
After Charles died in February 1660, his son made peace with his opponents. Sweden kept most of its gains from Denmark, but the belligerents largely returned to the status quo ante bellum.
The 1648 Peace of Westphalia that ended the Thirty Years' War confirmed Sweden as a major European power, and the replacement of its rival Denmark-Norway in the Baltic Sea. While Sweden and Russia had been at peace since the Treaty of Stolbovo in 1617, the former remained at war with the Polish–Lithuanian Commonwealth, although a truce between the two sides had been continually renewed since 1630.
The Commonwealth had been increasingly weakened by internal rebellions, such as the Khmelnytsky Uprising, while conflict between King John II Casimir Vasa and the Sejm paralysed the administration. The Russo-Polish War (1654–67) began when Russian troops invaded Poland, supported by Cossack rebels. Meeting little opposition, these soon neared the Baltic coast, potentially threatening Swedish interests in the region.
This drove Charles X Gustav of Sweden to intervene, claiming to be protecting fellow Protestants in Poland. He sought support from the Cossack Hetmanate, promising military support if they abandoned the Russians. Although Sweden was anxious to avoid direct conflict with Russia, negotiations with Poland broke down when John II Casimir refused to drop claims to the Swedish crown. Combined with the unwillingness of the Polish–Lithuanian nobility to make concessions, talks in Lübeck ended in February 1655 without a result. Sweden now decided to launch a preemptive attack on the Commonwealth, seeking to occupy its yet available territories before the Russians.
Swedish campaigns in the Polish–Lithuanian Commonwealth
Swedish forces entered Poland–Lithuania from Swedish Pomerania in the west, and Livonia in the north. The division on the western flank consisted of 13,650 men and 72 artillery pieces commanded by Arvid Wittenberg who entered Poland on 21 July 1655 and another 12,700 to 15,000 commanded by Charles X Gustav who followed in August, while the division on the northern flank consisted of 7,200 men commanded by Magnus De la Gardie who had already seized Dünaburg with them on 12 July.
On the western front, Wittenberg was opposed by a Polish levy of 13,000 and an additional 1,400 peasant infantry. Aware of the military superiority of the well-trained Swedish army, the nobles of Greater Poland surrendered to Wittenberg on 25 July in Ujście after the Battle of Ujście, and then pledged loyalty to the Swedish king. Wittenberg established a garrison in Poznań (Posen).
On the northern front, Prince Janusz Radziwiłł signed the Treaty of Kėdainiai with Sweden on 17 August 1655, placing the Grand Duchy of Lithuania under Swedish protection. Though Radziwiłł had been negotiating with Sweden before, during his dispute with the Polish king, Kėdainiai provided a clause stipulating that the two parts of the Commonwealth, Poland and Lithuania, need not fight each other. Part of the Grand Ducal Lithuanian Army opposed the treaty however, forming a confederation led by the magnate and Polish–Lithuanian hetman Paweł Jan Sapieha at Virbalis.
On 24 August, Charles X Gustav joined Wittenberg's forces. The Polish king John II Casimir left Warsaw the same month to confront the Swedish army in the west, but after some skirmishes with the Swedish vanguard retreated southwards to Kraków. On 8 September Charles X Gustav occupied Warsaw, then turned south to confront the retreating Polish king. The kings met at the Battle of Żarnów on 16 September, which like the next encounter at the Battle of Wojnicz on 3 October was a victory for Sweden. John II Casimir was exiled to Silesia while Kraków surrendered to Charles X Gustav on 19 October.
On 20 October, a second treaty was ratified at Kėdainiai in the north. The Union of Kėdainiai unified Lithuania with Sweden, with Radziwiłł recognizing Charles X Gustav as Grand Duke of Lithuania. Over the following days, most of the Polish army surrendered to Sweden: on 26 October Koniecpolski surrendered with 5,385 men near Kraków, on 28 October Field Crown Hetman Stanisław Lanckoroński and Great Crown Hetman Stanisław "Rewera" Potocki surrendered with 10,000 men, and on 31 October the levy of Mazovia surrendered after the Battle of Nowy Dwór.
Occupation of Poland–Lithuania and the Brandenburgian intervention
Meanwhile, Russian and Cossack forces had occupied the east of the Polish–Lithuanian Commonwealth as far as Lublin, with only Lwow (Lviv, Lemberg) remaining under Polish–Lithuanian control. In late October, Charles X Gustav headed northwards and left Wittenberg in Kraków with a mobile force of 3,000 Swedish and 2,000 Polish troops, and an additional number scattered in garrisons, to control the southern part of the Swedish-occupied commonwealth.
In the north, the Royal Prussian nobles concluded a defensive alliance with the Electorate of Brandenburg on 12 November in the Treaty of Rinsk, permitting Brandenburgian garrisons. Danzig (Gdansk), Thorn (Torun) and Elbing (Elblag) had not participated in the treaty, with Thorn and Elbing surrendering to Sweden. In the Treaty of Königsberg on 17 January 1656, Frederick William, Elector of Brandenburg and Duke of Prussia, took the Duchy of Prussia, formerly a Polish fief, as a fief from Charles X Gustav. The Brandenburgian garrisons in Royal Prussia were withdrawn, and when Marienburg (Malbork) surrendered in March, Danzig remained the only town not under Swedish control.
The rapid Swedish invasion and occupation of the Polish–Lithuanian territories became known in Poland as the "(Swedish) deluge."
The "deluge" and religious differences between the primarily Protestant Swedes and the primarily Catholic Poles, resulting in cases of maltreatment and murder of Catholic clergy and monks as well as cases of looting of Catholic churches and monasteries, gave rise to some partisan movements in the Swedish-occupied territory. A guerilla force attacked a small Swedish garrison at Koscian in October 1655 and killed Frederick of Hesse, brother-in-law of the Swedish king. The Pauline monastery Jasna Góra in Częstochowa successfully resisted a Swedish siege throughout November 1655 to January 1656. On 20 November a manifesto was issued in Opole (Oppeln) calling for public resistance and the return of John II Casimir, and in December a peasant force took Nowy Sącz. On 29 December, the partisan Tyszowce Confederation was constituted under participation of Lanckoroński and Potocki, and on 1 January 1656 John II Casimir returned from exile. Later in January, Stefan Czarniecki joined in, and by February most Polish soldiers who were in Swedish service since October 1655, had switched sides to that of the confederation.