The First Schleswig War (German: Schleswig-Holsteinischer Krieg), also known as the Schleswig-Holstein uprising (German: Schleswig-Holsteinische Erhebung) and the Three Years' War (Danish: Treårskrigen), was a military conflict in southern Denmark and northern Germany rooted in the Schleswig–Holstein question: who should control the Duchies of Schleswig, Holstein and Lauenburg, which at the time were ruled by the king of Denmark in a personal union. Ultimately, the Danish side proved victorious with the diplomatic support of the great powers, especially Britain and Russia, since the duchies were close to an important Baltic seaway connecting both powers.
While Schleswig had been predominantly Danish with a German elite concentrated in the cities and estates, modernisation brought extensive German influence. German became the language of administration, education, and the church, creating an environment in which it was omnipresent and speaking Danish offered no advantages. Due to top-down pressure, many Danish South Schleswigers gradually adopted German in their daily lives. This linguistic shift led to the extinction of the two Danish South Schleswig dialects, Fjoldemål and Angeldanish – first in favour of Low German, and later High German. Over time, these now Low German-speaking South Schleswigers became fully Germanized, came to identify with the German question, and sought to separate Schleswig, along with Holstein, from Denmark entirely. Therefore, by the outbreak of the war, Schleswig was divided: a Danish-speaking North and a Low German-speaking South, while Holstein and Lauenburg were entirely ethnically German.