The war with Austria came at a fairly high cost though. Just at the (decisive) battle of Koniggratz, (involving the largest number of combatants in Europe until that time), ended with an extremely high casualty rate. For the Prussians it was nearly 9,000 men killed, wounded or missing. The Austrians and thier allies had over 44,000 men killed, wounded or missing, with 22,000 of these being prisoners. What made the losses for the Austrians higher was that Austria had refused to sign the First Geneva Convention. Because of this, their medical personnel were regarded as combatants, and withdrew from the field with the main bulk of the forces, leaving wounded to die on the field. Battle after battle, regardless of the sides, ended up in bloodbaths for the Austrians--in some cases, whole battalions being lost. On the Austrian side, they often suffered 10 times the number of casualties as the Prussians. In one battle, they lost 3000 men in one hour. I think the final totals were about 71000 killed or wounded for the Austrians and their allies and 37000 for the Prussians and theirs.
This war (to go back to the main topic) effectively ended the monarchy in Hannover, which had sided with the Austrians. Not even the relationship between Hannover & England was enough to spare the King his throne. Those German states that sided with Austria included the Kingdoms of Saxony, Bavaria, Württemberg, and Hanover. Southern states such as, Baden (home of the Kaiser's son-in-law), Hesse-Kassel, Hesse-Darmstadt (home of the Crown Princess's sister Alice), and Nassau as well as Saxe-Meiningen, Reuss-Greiz, and Schaumburg-Lippe also joined with Austria. Some of those siding with Prussia, included Mecklenburg-Schwerin (home of the Kaiser's brother-in-law), Mecklenburg-Strelitz, Oldenburg, Anhalt, Brunswick, Saxe-Altenburg, Saxe-Coburg and Gotha and Waldeck-Pyrmont.
For those interested in the topic, there's a book The Austro-Prussian War: Austria's War with Prussia and Italy in 1866. Blending military & social history, it basically breaks down the campaigns, battle by battle.