The First Northern War (also Second or Little Northern War) was a conflict that took place from 1655 to 1661 between Poland, Sweden and Russia for supremacy in the Baltic states. Brandenburg fought initially on the side of Sweden against Poland, but changed sides, after Poland granted its prince-elector sovereignty over the Duchy of Prussia in the Treaty of Wehlau on 19 September 1657. Brandenburg succeeded in gaining ultimate sovereignty over the Duchy of Prussia and proved itself during the war as an important military and political power.