During the reign of Karl X Gustav (1654-1660), Sweden was a superpower which had at its disposal a superb army made-up of Swedish and Finish soldiers as well as of mercenaries recruited from places like Germany.
Karl X Gustav, an outstanding commander and an advocate of strong royal power, continued the aggressive policy of his predecessor Gustav II Adolph, and strove to increase his control over the Baltic. This cornerstone of this policy, the strengthening of the armed forces and the emergence of an anti-Polish political course, can be found as early as 1652 during the reign of Queen Kristine and war with either Moscow or the Polish-Lithuanian Commonwealth was planned before 1655. Finally the decision was made to attack the Polish-Lithuanian Commonwealth, which was facing serious trouble (wars at the East with the Cossacks and Muscovites) and its fall seemed imminent. Besides the king of Sweden thought that he may count on the support of Polish magnates. He was assured of this by a Polish nobleman, named Radziejowski, who was living in Sweden. King Jan Kazimierz was not popular and the first months of the war seemed to confirm these assumptions – the nobility easily agreed for Swedish protection. The capitulation of Polish forces under Uście (Wielkopolska) and in Kiejdany (duke Radziwiłł) were telling examples of this. The latter event still arouses controversies whether this was a clear treason, or a necessity by which duke Janusz Radziwiłł tried to save Lithuania from being overran by Muscovites. Only the following events – the growing anti-Swedish resistance, the return of King Jan Kazimierz and Swedish policy of pillage in the conquered areas - turned the fate of the war. Other important elements was the Swedish occupational policy and religious motives (Swedish were Lutheran). Swedish historians emphasize that at the beginning Karl X Gustav planned to conquer only Northern Poland and it was only after his initial successes that he expanded his plans trying to take the whole of the Commonwealth. The Swedish had substantial forces for their primary target of conquering the coastal areas of the Polish-Lithuanian Commonwealth. However, their forces turned out to be too weak for the taking the whole territory.
The power of Sweden was so great at that time that it was able to wage war on several fronts against the Muscovites, Poles and Danes. At the moment of the outbreak of the war with Poland Karl X Gustav had about fifty five thousand soldiers, many of them veterans, with a well organized and equipped artillery. The war in Poland was very unpopular and brought many losses to the Swedes as a result of guerilla actions. Swedish forces were “disappearing” in the Polish-Lithuanian Commonwealth and from 1657 Karl X Gustav only thought about ways to withdraw from the war without loosing face.
From 1657 Karl X Gustav was engaged in a war with Denmark (in fact two Danish wars), judging that chances for success there were better than in Poland. From this moment on the king of Sweden started to treat the war with the Polish-Lithuanian Commonwealth as a secondary operation and did not appear in Poland anymore in the years 1658-1660. The attack on Denmark started a process of creation of a strong anti-Swedish coalition (Denmark, the Holy Roman Empire, the Polish-Lithuanian Commonwealth, Brandenburg, the Netherlands, Moscow). Imperial and Brandenburg forces as well as a division of Poles under Czarniecki marched off to Denmark, where the main fighting took place. By the end of the “deluge” Sweden was pushed into defensive, and there were no more chances for a victory. Eventually, Sweden could not oppose a broad coalition and they suffered defeats in Pomerania and Denmark. The peace treaty was signed in Oliwa in May 1660.
Swedish war-machine had no match in mid seventeenth century and was considered very aggressive and effective. The disadvantage of Swedish military was the fact that a country with a small population had to wage war relying on mercenary troops. This meant waging wars on credit – funds were necessary to pay off the mercenaries and to get spoils another victorious war was needed. As a result, Poland was plundered to fill the Swedish treasury which was constantly empty due to the huge cost of the war. The losses in male population of Sweden suffered during the wars of Karl X Gustav were very painful.