Once he returned to Brandenburg, Frederick William realized that the Swedish forces, occupying the swampy Havelland region between Havelberg and the town of Brandenburg, were dispersed and ordered Derrflinger to take the central town of Rathenow in order to split them roughly down the middle.
The town of Rathenow was also occupied by Swedish troops, because Wrangel wanted to launch a crossing of the River Elbe at Havelberg from Rathenow and join forces with Hanoverian troops.
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As a consequence of this setback, the Swedes had to abandon their plan to cross the Elbe at Havelberg in the vicinity of Rathenow, in order to attack the key Brandenburg fortress of Magdeburg.
He remained committed to that theatre through 1905, portraying such roles as Walitzin/Commissario in Umberto Giordano's Siberia and Johannes Rathenow in the Italian premiere of Ruggero Leoncavallo's Der Roland von Berlin.
In 1820 he earned his doctorate from the Friedrich Wilhelm Institute in Berlin, and spent the next several years as a general practitioner in the cities of Bernau, Rathenow and Genthin.