Tonight is a Historical Hetalia doodle night apparently
Anyway, this is inspired by a bit of reading I did on the Treaty of Marienburg in 1656 signed between King Charles X Gustav of Sweden and Elector Frederick William of Brandenburg during the Second Northern War. By the Treaty of Königsberg in the same year, Ducal Prussia was officially made a fief of the invading Swedish forces. In exchange for military aid, King Charles promised Frederick William control over parts of Prussia. The alliance held out for some time, culminating in a victorious battle against Polish-Lithuanian troops by combined Swedish and Brandenburgian forces.
When Sweden found itself more dependent on Brandenburg’s support to win the war, Charles was willing to grant Frederick William full sovereignty over the Duchy in exchange for continuous aid. This agreement was solidified by the Treaty of Labiau in the same year. Once he managed to secure his possession of the Duchy of Prussia, Frederick William dropped his alliance with Sweden in favour of a more appealing proposal put up by an envoy of the Holy Roman emperor. If he were to stop supporting Sweden, Poland would acknowledge Hohenzollern sovereign rule over Prussia. Brandenburg accepted these terms by signing the secret Treaty of Wehlau and actively started campaigning against Sweden.
TLDR; Reiner Wenzel Brandt, Margraviate of Brandenburg, is one crafty son of a bitch.