(1866), war between Prussia on the one side and Austria, Bavaria, Saxony, Hanover, and certain minor German states on the other. It ended in a Prussian victory, which meant the exclusion of Austria from Germany. The issue was decided in Bohemia, where the principal Prussian armies met the main Austrian forces and the Saxon army, most decisively at the Battle of Königgrätz (Königgrätz, Battle of) (q.v.). A Prussian detachment, known as the army of the Main, meanwhile dealt with the forces of Bavaria and other German states that had sided with Austria. Simultaneously, a campaign was fought in Venetia between the Austrian army of the south and the Italians, who had made an alliance with Prussia.

The 1866 campaign was a carefully planned stage in the unification of Germany under Prussia's Hohenzollern dynasty, of which Otto von Bismarck (Bismarck, Otto von) was the principal agent. The issue was clear-cut: Prussia deliberately challenged Austria for the leadership of the German Confederation. Prussia had challenged Austria in 1850, but the complete failure of its mobilization in that year compelled the acceptance at Olmütz of the somewhat humiliating terms of Austria. Since then Prussia, with Bismarck as statesman, Count Helmuth von Moltke as strategist, and Count Albrecht von Roon as army organizer, had prepared methodically for a fresh challenge. The actual pretext found by Bismarck in 1866 was a dispute over the administration of Schleswig and Holstein, which Austria and Prussia had seized from Denmark in 1864 and had since held jointly. Diplomatic exchanges began in January and military preparations a little later, but hostilities did not actually break out until the middle of June.

By the alliance with Italy, Bismarck contrived to divert part of the Austrian forces to the south. This advantage, together with that of Prussia's modernized army discipline, resulted in a Prussian victory; the war was formally concluded on August 23 by the Treaty of Prague. The treaty assigned Schleswig-Holstein to Prussia. The latter also annexed Hanover, Hesse-Kassel, Nassau, and Frankfurt outright, thus acquiring the territory that had separated the eastern and the western parts of the Prussian state. By the Peace of Vienna (Oct. 3, 1866) Austria ceded Venetia for transfer to Italy. Prussia's victory in the war enabled it to organize the North German Confederation.