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Why did Frederick William first ally Brandenburg with Sweden during the First Northern War?
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Frederick William
Frederick William, undated engraving.
During the First Northern War (1655–1660), Frederick William did not initially ally with Sweden by choice; rather, he attempted to maintain a policy of armed neutrality when King Charles X Gustav of Sweden first offered an alliance in exchange for control over East Prussian ports.1
The Elector was forced to abandon this neutrality only after Charles Gustav rapidly overran Poland and advanced directly against East Prussia. Faced with this military pressure, Frederick William was compelled to exchange Polish suzerainty for Swedish suzerainty and provide the Swedish king with armed support.1