The Austro-Prussian War, also called the Brothers War, was the conflict that led to Prussia forcing Austria to acknowledge its hegemony in the German-speaking world, thus allowing the former to create 
Germany. The victor of a Brother’s War in Victoria 2 is normally Prussia, due to their superior military and large amounts of allies which acts as a “vassal swarm”.
Typically in the game, the two countries are supported by the states in their spheres of influence:
Saxony is often on Austrias side, but can be seen on the other side as well. 
Modena and other Italian states sphered by Austria as well as 
Krakow can also take a part on Austria's side.