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In 1797, the French returned. The army of Napol�on Bonaparte occupied the German territory to the west of the Rhine river, and the Treaty of Campo Formio awarded France this entire area. On 17 February 1800, the French D�partement du Mont-Tonnerre was founded here, with Mainz as its capital, the Rhine river being the new eastern frontier of la Grande Nation. Austria and Prussia could not but approve this new border with France in 1801. However, after several defeats in Europe during the next years, the weakened Napol�on and his troops had to leave Mainz in May 1814.
In 1816, the part of the former French D�partement which is known today as Rhenish Hesse (German: Rheinhessen) was awarded to the Hesse-Darmstadt, Mainz being the capital of the new Hessian province of Rhenish Hesse. From 1816 to 1866, to the German Confederation Mainz was the most important fortress in the defence against France, and had a strong garrison of Austrian and Prussian troops.
In the afternoon of 18 November 1857, a huge explosion rocked Mainz when the city's powder magazine, the Pulverturm, exploded. Approximately 150 people were killed and at least 500 injured; 57 buildings were destroyed and a similar number severely damaged in what was to be known as the Powder Tower Explosion or Powder Explosion.
During the Austro-Prussian War in 1866, Mainz was declared a neutral zone. After the founding of the German Empire in 1871, Mainz no longer was as important a stronghold, because in the war of 1870/71 France had lost the territory of Alsace-Lorraine to Germany, and this defined the new border between the two countries.
For centuries the inhabitants of the fortress of Mainz had suffered from a severe shortage of space which led to disease and other inconveniences. In 1872 Mayor Carl Wallau and the council of Mainz persuaded the military government to sign a contract to expand the city. Beginning in 1874, the city of Mainz assimilated the Gartenfeld, an idyllic
