In a dispute over Schleswig and Holstein, the German Confederation’s federal assembly (Bundestag) approved mobilization against Prussia. Prussia treated the decision as unlawful and used it to justify breaking with the Confederation’s existing order. This vote set the political trigger for war and opened the path to coercive territorial change.
Within days of the mobilization vote, Prussian forces moved against states aligned with Austria in the German Confederation. This rapid escalation turned the political crisis into open conflict in central Germany. The campaign quickly brought Hanover and Hesse-Kassel under direct military pressure.
Prussian troops occupied Kassel, the capital of the Electorate of Hesse (Hesse-Kassel). Elector Frederick William was taken into custody, removing the local ruler as an active political and military actor. Control of Kassel helped Prussia secure a strategic corridor in central Germany.
Hanover’s army won a tactical victory at Langensalza, but the wider situation still favored Prussia. Prussian forces were able to concentrate and cut off Hanover’s routes to potential allies. The battle mattered because it led directly to Hanover’s loss of freedom of action and soon to surrender.
After Langensalza, Hanover’s forces found themselves encircled and short of supplies. The Hanoverian army accepted capitulation terms, ending effective military resistance. This collapse made it easier for Prussia to impose political annexation on the kingdom.
Prussian forces entered the Free City of Frankfurt, which had remained loyal to the German Confederation rather than joining Prussia. Occupation brought immediate administrative control and heavy financial demands on the city. Frankfurt’s capture became a key step toward ending its status as an independent city-state.
Prussia and Austria signed the Peace (Treaty) of Prague, effectively ending the Austro-Prussian War. The settlement dissolved the German Confederation and accepted Prussia’s new leading role in northern Germany. It also cleared the way for Prussia to annex several defeated German states outright.
King Wilhelm I and the Prussian Landtag enacted a law permanently uniting Hanover, Hesse-Kassel, Nassau, and Frankfurt with the Prussian monarchy. The law set a timetable for extending the Prussian constitution to the annexed territories. This was the central legal act turning wartime occupation into formal incorporation.
By royal decree, Prussia ended Frankfurt’s independence and attached it to Prussia. The decision removed one of the German Confederation’s most prominent free cities and reshaped regional trade and politics. Frankfurt would later be administered within Prussia’s new structures in the region.
Civil and military ceremonies marked the practical completion of the annexations in Hanover and in Kassel, Wiesbaden, and Frankfurt. These events signaled the shift from temporary occupation to settled Prussian rule on the ground. The ceremonies also helped Prussia present annexation as a finished political fact to local populations and other states.
The North German Confederation came into existence as a new federal state led by Prussia. Prussia’s 1866 annexations strengthened its geographic and political position inside this new system. The Confederation became the immediate institutional bridge from regional annexations to a wider northern German union.
The annexation law provided that the Prussian constitution would come into force in the annexed lands on this date. Extending Prussian constitutional and administrative rules standardized governance and reduced the legal separation between old and new Prussian territories. This was a key step in turning annexed states into normal parts of the Prussian political system.
After annexation, Prussia moved toward a stable provincial structure for its new central German lands. The Province of Hesse-Nassau was formed to group Hesse-Kassel, Nassau, and Frankfurt under one provincial administration. This reorganization helped Prussia govern the annexed territories more efficiently and tied them more tightly to Berlin.
Prussian annexations of German states after 1866: Hanover, Hesse‑Cassel, Nassau and Frankfurt (1866–1867)