Schleswig-Holstein Question
As German nationalism rose in the mid-1800s, the Schleswig-Holstein Question came to dominate Denmark's foreign affairs. After the First Schleswig War Russia took Denmark's side in the negotiation of the 1852 London Protocol, which reaffirmed Danish sovereignty in the disputed duchies. Russia also offered diplomatic support for Denmark in the Second Schleswig War but did not make a military intervention, and Denmark ultimately lost Schleswig-Holstein in its entirety to Prussia in the 1864 Treaty of Vienna.