Polish-Swedish Wars

The Polish–Swedish Wars were a series of wars between the Polish–Lithuanian Commonwealth and Sweden. Broadly construed, the term refers to a series of wars between 1563 and 1721. More narrowly, it refers to particular wars between 1600 and 1629. These are the wars included under the broader use of the term:

The conflict between Polish–Lithuanian Commonwealth and Sweden can trace its roots to the War against Sigismund, where Sigismund III Vasa, at one time king of both the Commonwealth and Sweden, lost the throne of Sweden during the civil war (1597–1599). Few Commonwealth troops participated in this conflict, and it is mostly regarded as a Swedish civil war, not part of the Polish–Swedish wars. After early stalemate, Sigismund was defeated in the Battle of Stångebro in 1598 and by 1599, Sigismund was dethroned by his uncle, Duke Charles and forced to retreat to the Commonwealth. This also spelled the end of the short-lived personal union between Poland and Sweden.

However, even after being dethroned, Sigismund did not give up on regaining the Swedish throne, and from then on, most of his policies would revolve around his attempts to conquer Sweden, even though Commonwealth nobility had little will for such a long and bloody conflict. Sigismund started his plan in 1599, when he confirmed the pacta conventa, documents with promises he signed when elected as a King of Poland, which stated that the then-Swedish territory of Estonia would now became part of the Commonwealth. Polish nobility, the szlachta, supported this particular conflict, assuming it would be limited to Estonia only, and expecting many gains in form of new lands and increases of grain export through access to Estonian ports on the Baltic Sea. In addition, szlachta did not think highly of the Swedes, and did not expect this war to drag long or be difficult. They grossly underestimated their opponent, thinking that Poland, having been nearly undefeated in battle for over a hundred years, would be easily able to parry any attacks of the Scandinavians. The Commonwealth had nearly 10 million inhabitants, almost 10 times that of 1 million in Sweden. On the other hand, szlachta forgot that the Commonwealth had one of the smallest military to population ratios in Europe, and did not know that the Swedish army was highly trained and motivated.

...
Wikipedia