Prussia rose high and fell hard. What was in its heyday a great power of Europe is now just another constituent kingdom of the German Federal Empire. The Prussians reached great heights under Frederick the Great, only to be smacked down by Napoleon. Although Napoleon was defeated, Prussia would never really recovered, and they were ultimately defeated by the Austrians and incorporated into the new German Empire. Your challenge, then, is to prevent Prussia from declining; have it remain a strong country, and maybe, as crazy as it sounds, have Prussia unify Germany.