DANISH WAR, the 1864 conflict between Denmark and the two German great powers that set the stage for the decisive clash between Austria and Prussia two years later. The root cause was the long-simmering rivalry between German and Danish nationalisms over the duchies of Schleswig and Holstein, historically attached to the Danish crown. Traditionally, France was a guarantor of Denmark's territorial integrity, but it was a Danish initiative that, at the end of 1863, challenged the status quo (sanctioned by the great powers at London in 1852) by seeking to apply one constitution to all possessions of the Danish crown, including Schleswig and Holstein. The Prussian president of council, Otto von Bismarck, chose to champion the 1852 Treaty of London and to this end entered into an alliance with Austria. Napoleon III opted for neutrality, for many reasons: the weakness of Denmark's case when measured by the principle of nationalities (only northern Schleswig was ethnically Danish); Bismarck's skillful tack of defending a treaty that all the great powers had signed; the French emperor's disappointment at British failure the preceding year to act vigorously in defense of Poland against Russian violations of the 1815 treaties and to support the French proposal ( November 1863) for a general European congress; and, perhaps, the sheer difficulty of acting alone against the combined strength of Prussia and Austria. British actions (or, rather, inaction) soon confirmed that no material help for Denmark could be expected from that quarter.
The war began 1 February 1864 with Prussia's invasion of Schleswig, France having rejected the previous month London's proposal for a conference. The key Danish position of Düppel (Schleswig) fell to Prussian assault 18 April, and the Danes were constrained to attend a conference that, this time with French support, met at London 25 April. Although Napoleon III proposed a division of the duchies along national lines, the Danes remained obdurate, and the conference ended with Prussia and Austria demanding cession (28 May). The war recommenced 26 June. Defeated, Denmark was compelled to accept an armistice and, on 1 August, peace preliminaries, by which it surrendered Schleswig-Holstein to the victorious allies. The Treaty of Vienna of 30 October 1864 ended the Danish War. By consigning the conquered duchies to an Austro-Prussian condominium, this treaty may be said to have initiated events that would lead to