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Belligerents and Participants
AAfter Russia lost its Baltic fleet in the Russo-Japanese War, Germany became the dominant naval power in the Baltic Sea by default. Maintaining this supremacy was of high concern to the German admiralty during the Great War and Denmark held the key. Passage between the Baltic and the North Sea was controlled through her narrow straits. If mined they could keep the British from entering the Baltic, while the Germans would still have access to the North Sea through the Kiel Canal. In 1912, Denmark had to placate Britain by promising never to obstruct these waterways, but now under pressure from Germany, laying sea mine belts become the kingdom’s major military activity. Mine fields would be maintained in the straits between Jutland and the island of Funen, and the passageways between Funen and Sweden, which would be anchored at the garrisoned island of Sealand.
With few resources available to devote to this region, it was essential to Germany that Denmark stay out of the War, a policy that suited the Danes as well as King Christian X who had no interest in getting embroiled in this conflict. When the War broke out he declared Denmark neutral, but mobilized the largest army the Kingdom had ever seen as a precaution. Even so, it was obvious they could not defend their entire border against a determined attack, so most troops wound up playing a purely defensive role by garrisoning the massive fortifications that surrounded Copenhagen. While the German Navy showed some interest in attacking Denmark, the Army had no troops to spare for such operations. As it was, their shared border could only be lightly defended and it was Germany that felt the most threatened by the possible opening of a new front by Danish or British forces. Even if contained, offensive operations here could close the strategically important Kiel Canal.
Aggressive actions by Britain became a real possibility after Germany launched its unrestricted U-boat campaign. In response to a possible invasion through Denmark, the Germans began construction of a defensive line in 1916 to run across southern Jutland. If built strong enough, they figured this border could be held by fewer troops. By 1918 the Sicherungsstellung Nordhere ran through Schleswig-Holstein, stretching from the Island of Roemoe on the Wadden Sea eastward to Lillebalt. By the end of the War these elaborate defenses had not yet been completed in depth and were never fully manned.
Schleswig-Holstein had been seized by Prussia during the Second Danish-Prussian War in 1864 and eventually became part of the German Empire. While the southernmost region was dominated by Germans, Denmark disputed control over Schleswig, which had a more mixed population. Many of the Danes living here were the only ones to fight in the Great War after being conscripted into the German Army. The end of the War did not allay fears that Germany would look to reunite with ethnic Germans in Denmark, so the Treaty of Versailles called for a popular referendum where the people living there would decide which nation they wished to belong. Most of Schleswig opted for Denmark and the border was shifted southward. When the King joined Danish nationalists in demanding more territory, it brought him into open conflict with the Prime Minister. This brought about the Easter constitutional crisis of 1920 that eventually diminished the power of the monarchy. The shifting of the border also left all the German fortifications within Denmark but they were not oriented towards the Kingdom’s defense. The Danes began the destruction of these defensive works in 1921.