Complex sentence: Prussia only participated in one battle, the Battle of Stresow on the island of Rügen, as the war had already been practically decided in the 1709 Battle of Poltava.
Split into simpler sentences: Prussia only participated in one single battle during the war, the Battle of Stresow on the island of Rügen. The war had already been practically decided in the 1709 Battle of Poltava.