What role did Frederick William play in the First Northern War?

During the First Northern War (1655–1660), Frederick William, Elector of Brandenburg and Duke of Prussia, navigated a complex path of shifting alliances to secure his territorial interests. Initially attempting armed neutrality, he was eventually compelled to support the Swedish invasion of Poland, providing troops to King Charles X Gustav and participating in the Battle of Warsaw in 1656.12

As Sweden’s military position weakened, Frederick William strategically pivoted, entering into negotiations with Poland. This maneuver led to the Treaty of Wehlau, in which he agreed to support Poland against Sweden in exchange for the Polish king’s recognition of his full sovereignty over the Duchy of Prussia.13

Following this, he joined forces with Poland and the Habsburg emperor to expel Swedish troops from western Pomerania. Although French intervention ultimately forced him to relinquish his Pomeranian conquests, the Treaty of Oliva (1660) confirmed his hard-won sovereignty over the Duchy of Prussia, marking a significant step in the rise of Brandenburg-Prussia.13