After the 1815 Congress of Vienna, the Duchy of Holstein became a member of the German Confederation, resulting in several diplomatic and military conflicts about the so-called Schleswig-Holstein Question. Denmark defended its rule over Holstein in the First Schleswig War of 1848-51 against the Kingdom of Prussia. However, in the Second Schleswig War (1864) Prussian and Austrian troops conquered Schleswig. Christian IX of Denmark had to renounce both Schleswig and Holstein in the Treaty of Vienna (1864) on October 30. At first placed under joint rule in a condominium, Prussia and Austria then assumed administration of Schleswig and Holstein, respectively, under the Gastein Convention of August 14, 1865. However, tensions between the two powers culminated in the Austro-Prussian War of 1866. Following the Peace of Prague (1866), the victorious Prussians annexed both Schleswig and Holstein by decree of December 24, 1866, and later established the unified Province of Schleswig-Holstein.