The rule of the Dukes of Nassau in Wiesbaden came to an end with a "patent for taking possession of the former Duchy of Nassau" signed by King Wilhelm of Prussia at Babelsberg Palace near Berlin on October 3, 1866.

This was preceded by a military conflict between Prussia and Austria, which had ostensibly arisen due to disagreements between the two states over the joint administration of Schleswig-Holstein. Prussia was seeking to annex this territory and had already concluded a secret offensive alliance with Italy against Austria in April 1866 in breach of the treaty on which the German Confederation was based. The real battle was for Prussian supremacy in Germany, which Bismarck wanted to decide in favor of the Prussian crown at all costs - by war if necessary.

Just three weeks after the start of the war, the Prussians achieved a decisive victory at the Battle of Königgrätz on July 3, 1866, which culminated in the Preliminary Peace of Nikolsburg on July 26, 1866. In the final Peace of Prague on 23.08.1866, Austria agreed to the dissolution of the German Confederation and the incorporation of several small and medium-sized German states, including Nassau, which had fought on the Austrian side, by Prussia.

Prussian troops had already invaded northern Nassau on July 28, 1866 and temporarily occupied Montabaur, Dillenburg, Herborn and Ems. After the Battle of Königgrätz, Bismarck ordered the Prussian Main Army to occupy the entire duchy. Duke Adolph zu Nassau left Wiesbaden on July 15. The compensation negotiations with Prussia dragged on for over a year. In the compensation agreement of September 18, 1867, he was guaranteed ownership of the palaces in Biebrich and Weilburg, the Platte hunting lodge near Wiesbaden, the Paulinenschlösschen and some forest property. In addition, he received financial compensation of over 15 million fl. Wiesbaden was occupied by Prussian troops on July 18, 1866.

The annexation of Nassau met with little opposition from the population. On July 29, the former district administrator of Wetzlar, Gustav von Diest, took over the Nassau administration as civil commissioner and was able to promote the Prussian monarchy with confidence-building measures. At the ceremonial proclamation of the takeover on October 9, 1866, Prussian and Nassau flags flew side by side on Schillerplatz.