1866, War between Austria an Prussia/Austro-Prussian war/Unification war/...

1866 Austria declared war on Prussia. April 1866, the Prussian representative in Florence signed a secret agreement with the Italians. This committed the two states to assist each other in a war against Austria. The next day, the Prussian delegate to the Frankfurt assembly presented a plan calling for a national constitution.

Situation at the time of the outbreak of the war:

  Prussia

  Austria

  Austria's allies

  Prussia's allies

  Neutral

  Under joint administration (Schleswig-Holstein)

The trigger of this war was the conflict between Austria and Prussia about the administration of        Schleswig-Holstein. Prussians allies were mainly Italy but also the Großherzogtum Oldenburg, Hamburg, Bremen, Lübeck and many more.
After Prussians victory at the first battle the Treaty of Prague was signed (23th August 1866), which formally concluded the war. The  treaty assigned Schleswig-Holstein to Prussia.
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 excluding Austria.