Austria and Prussia were the most powerful states in the Holy Roman Empire by the 18th and 19th centuries and had engaged in a struggle for supremacy in Germany. The rivalry was characterized by major territorial conflicts and economic, cultural and political aspects. Therefore, the rivalry was an important element of the so called German question in the 19th century. Both opponents first met in the Silesian Wars and Seven Years' War during the middle 18th century until the conflict's culmination in the Austro-Prussian War of 1866. The German term is (literally German dualism), which does not cover only rivalry but also cooperation, for example in the Napoleonic Wars. Indeed, both powers did jointly dominate the German Confederation which functioned only in times of cooperation (1815-1848 and 1851-1859). They still fought on the same side (against Denmark) in the Second Schleswig War (1864). After 1866 (North German Confederation) and 1871, the new German nation state was dominated by Prussia. As Austria (or Austria-Hungary, since 1867) no longer struggled over the hegemony in Germany, the term Deutscher Dualismus became meaningless. Germany and Austria-Hungary soon became close allies, as proven by the Zweibund of 1879. Both countries were the main Central Powers during World War I (1914-1918). After that war, Austria-Hungary fell apart, and Germany became a republic. Background The Margraviate of Brandenburg was officially declared one of the seven electorates of the Holy Roman Empire by the Golden Bull of 1356. It had extended most of its territory into the eastern Neumark region, and after the War of the Jülich succession by the 1614 Treaty of Xanten also gained the Duchy of Cleves as well as the counties of Mark and Ravensberg located in northwestern Germany. It finally grew out of the Imperial borders when in 1618 the Hohenzollern electors became dukes of Prussia, then a fief of the Polish Crown, and the lands of Brandenburg-Prussia were ruled in personal union. In 1653, the "Great Elector" Frederick William acquired Farther Pomerania and reached full sovereignty in Ducal Prussia by the 1657 Treaty of Wehlau concluded with the Polish king John II Casimir Vasa. In 1701, Frederick William's son and successor Frederick I reached the consent of Emperor Leopold I to proclaim himself a King "in" Prussia at Königsberg, with respect to the fact that he still held the electoral dignity of Brandenburg and the royal title was only valid in the