The Prussian victory in the 1866 Austro-Prussian War destabilized the European balance of power. German unification, although not yet quite complete, posed a serious threat to French power. The candidacy of Prussian Prince Leopold to the vacant throne of Spain further endangered the status quo created by the 1815 Congress of Vienna. Demands by Emperor Napoleon III of France to King William I of Prussia, although reasonable at first, were soon seen as the perfect pretext to escalate tensions between the two empires. Prussian Prime Minister Otto von Bismarck ensured such tensions did escalate with the release of the Ems Dispatch.