Political map of Europe & the Mediterranean 30 October 1864 (Second Schleswig War): The two most powerful German states were Austria and Prussia. After the German occupation of Holstein, they demanded that Denmark relinquish Schleswig (History of Schleswig-Holstein). When the Danes refused to back down, the Austrians and Prussians invaded (Second Schleswig War). By October Denmark had been overrun and forced to come to terms. (Treaty of Vienna (1864))