Due to its imperial significance, Frankfurt had survived mediatisation in 1803.

Following the collapse of the Holy Roman Empire in 1806, Frankfurt had fallen to the rule of Napoleon I, who granted the city to Karl Theodor Anton Maria von Dalberg; the city became known as the Principality of Frankfurt.

The Catholic clergy Dalberg had emancipated the Catholics living with the city boundary.

In 1810 Dalberg merged Frankfurt with the Principality of Aschaffenburg, the County of Wetzlar, Fulda, and Hanau to form the Grand Duchy of Frankfurt.

After the defeat of Napoleon and the collapse of the Confederation of the Rhine, Frankfurt had been returned to its pre-Napoleonic constitution via the Congress of Vienna of 1815 and has become a sovereign city-state and a member of the German Confederation.

During the period of the German Confederation, Frankfurt continues to be a major city.

The confederation's governing body, the Bundestag (officially called the Bundesversammlung, Federal Assembly) is located in the palace of Thurn und Taxis in Frankfurt's city center.

During the Revolutions of 1848, the Frankfurt Parliament is formed in an attempt to unite the German states in a democratic manner.

It is here that Prussian king, Frederick William IV, refuses the offer of the crown of "Little Germany".

In 1866 the Kingdom of Prussia goes to war with the Austrian Empire over Schleswig-Holstein, causing the Austro-Prussian War.

Frankfurt, remaining loyal to the German Confederation, does not join with Prussia.

Following Prussia's victory, Frankfurt is annexed by Prussia, becoming part of the newly formed province of Hesse-Nassau.