In 1672, the Franco-Dutch War broke out, with Brandenburg-Prussia involved as an ally of the Dutch Republic. This alliance was based on a treaty of 1669, and resulted in French occupation of Brandenburg-Prussian Cleves. In June 1673, Frederick William abandoned the Dutch alliance and concluded a subsidy treaty with France, who in return withdrew from Cleves. When the Holy Roman Empire declared war on France, a so-called "Reichskrieg", Brandenburg-Prussia again changed sides and joined the imperial forces. France pressured her ally Sweden to relieve her by attacking Brandenburg-Prussia from the north. Charles XI of Sweden, dependent on French subsidies, reluctantly occupied the Brandenburgian Uckermark in 1674, starting the German theater of the Scanian War (Brandenburg-Swedish War). Frederick William reacted promptly by marching his armies from the Rhine to northern Brandenburg, and encountered the rear of the Swedish army, which was in the process of crossing a swamp, in the Battle of Fehrbellin (1675). Though a minor skirmish from a military perspective, Frederick William's victory turned out to be of huge symbolic significance. The "Great Elector" started a counter-offensive, pursuing the retreating Swedish forces through Swedish Pomerania.