"...Denmark was, by European standards, a military minnow. It had a professional army that was modestly well equipped and with reserves could get close to two hundred thousand men in the field within twelve days; her land defenses were primarily a ring of forts defending western approaches across the island of Zealand towards Copenhagen, and a sturdy but aging defensive line running from Kolding to Esbjerg. As a country that was a combination of a long peninsula and an archipelago that controlled the entrance to the Baltic Sea, it naturally had much invested in the Royal Danish Navy, but the traumatic experience of the two Battles of Copenhagen a century earlier had dissuaded Denmark from building a grand fleet in being, especially in a time where her access to resources was highly controlled by foreign powers. Instead, Denmark had invested herself entirely in a flexible light vessel strategy that was meant to be able to close the Danish Straits; Denmark had not dreadnoughts and cruisers but rather a series of coastal defense ships with guns of excellent range, a large fleet of minelayers, and dozens of submarines modelled after the defensive Spanish Peral series of vessels. She was a popular recipient of outdated boats from France and Britain, and as the war kicked off, Denmark had the means and intention to close the Danish Straits entirely to German vessels.
German planning around Denmark, in the Prussian tradition, allowed for a tremendous amount of initiative and improvisation. Diplomats and generals in Berlin alike were skeptical as to whether Denmark actually would mobilize, and if they did whether it would be to defend their neutrality or with hostile intent. Jagow, perhaps more than anybody else, was entirely convinced that Denmark's liberal, pacifist Prime Minister Carl Theodor Zahle would not even mobilize; Falkenhayn, ever the skeptic, rather advised the Kaiser and Furstenburg that Germany was better off assuming that Denmark would bow to pressure and likely start mining the Danish Straits if nothing else. That this prediction turned out to be true with news of Danish mobilization on March 10th tilted the conversation in Falkenhayn's direction; when Danish mobilization was not just filling the forts at Copenhagen but also the Jutland Line, Germany included Denmark in her declaration of war.
However, German planners viewed Denmark as an obstacle rather than an enemy, and it ranked below Austria and certainly France and Belgium in the German hierarchy of hostility. The plan was to quickly overrun Danish defenses and cut off Zealand from the rest of occupied Denmark in order to force a negotiated peace. Germany allocated 12 divisions to this operation, ten of which were to cross via Jutland and another two that were to cross the Little Belt to Bojden on the island of Funen to rapidly march on Odense, the country's third-largest city. The main thrust of the German Jutland offensive, meanwhile, was meant to capture the small industrial triangle of Kolding, Fredericia and Vejle via overwhelming concentrated force at the eastern end of the Jutland Line and then continue pressing north, with the weight of German attackers ending their march at Aarhus while mop-up cavalry operations ended in Randers and Viborg. This would leave Germany in control of all of eastern Jutland's major towns and cities and, critically, its roads and railroads, and control of Odense would cut off Copenhagen from Jutland; the offensive ending where it did would also isolate Aalborg in the north. Due to the relatively low population of Denmark's west coast - flat, sandy, and marshy - this operation, known as Case D, would break the country in two by land and ferry.
Secondary to this operation, but no less important, was the Kaiserliche Marine rapidly seizing control of the Danish Straits, known as the Belts. The idea was to quickly establish supremacy at sea on both sides of Funen and thus be able to cut off Zealand entirely, ending the ability of Copenhagen to resupply Aalborg, and vice versa. Ideally, this would force Denmark's hand into surrender, because Falkenhayn was highly confident that Denmark would fold rather than see a German force attacking across Zealand..."
- The Reich at War
"...Germany attacked Denmark a whole day later than Austria, in part due to skepticism up until the last second that Denmark indeed wanted to poke the bear. But with the arrival of a Danish declaration of war alongside the French one, Case D was triggered, and the 3rd Army under Ludwig von Estorff launched their attack across the full seventy kilometer frontier, with seven divisions attacking the Kolding end of the Jutland Line and three divisions tying down the rest with mobile artillery and sporadic probing attacks.
On the 14th, Denmark was still only halfway into their mobilization and defenses around Vamdrup and Kolding were still undermanned; German troops quickly overwhelmed not just border checkpoints and fortified positions such as pillboxes and small open-aired forts but essentially marched into Kolding in force. A day later, the German Army had similarly seized Fredericia just twelve kilometers away, succeeding in quick fashion in achieving all of their second-day objectives. By the 16th, most Danish soldiers had regrouped and rallied towards the north, but the quick failure of the Jutland Line had essentially sealed their fate.
Key to the rapid German advance of March 14-20 across Jutland was, of course, their deployment of motorized personnel carrier and armored vehicles. Germany had been a skeptic of landship development due to the mountainous terrain they would face in a war with Austria or France, but Denmark's wide, open and flat land proved to be an important testing ground for motorized offensive theory that Prussian generals were eager to study, and some concepts of what would within a few decades be known as "combined arms" were deployed across Denmark. German light bombers and strafing craft attacked Danish positions in combination with artillery while German infantry could be moved rapidly in short distances by truck or armored personnel vehicle; the rudimentary but effective A7V and B9 landships were able to scatter Danish defenders and force frequent retreats. Every few hours, German soldiers would stop, regroup, and then attack again with artillery and aerial barrages timed well together and designed more to harrass and concern the enemy than pulverize them. Late in the evening of March 20, the German Army had driven into Aarhus, one day earlier than their offensive timetable demanded.
Military strategists have debated for decades the reasons for Jutland's quick fall. The Danes were a low spender on their army compared to other combatants but were clearly overwhelmed at Kolding with the speed, ferocity and innovation of Germany's offensive. Danish historians have faulted the Zahle government for a lack of preparation; German scholars, meanwhile, have been more complimentary, pointing out that Danish men fought bravely and doggedly all the way to Aalborg, but that the problem was that Danish military planning deployed over half of the standing army to defend the defensive lines towards Copenhagen and almost treated Jutland as a fait accompli in the case of a land war. This was not, on its own, an unreasonable strategy; even after the surrender of Denmark on March 24, Zealand remained untaken even if she lay blockaded, and the expense and effort it would have taken Germany to stage an amphibious invasion of the island and march on Copenhagen would have dramatically increased the damage and casualties for both parties, firmly against the political preferences of Berlin to execute a quick, relatively bloodless war on Denmark and knock them out of the conflict and into a pro-German camp once the war was over.
Of course, the quick march to Aarhus is just part of the story, and Case D was a tale of two offensives: the smashing success of a proto-combined arms strategy across Jutland harrying Danish defenders much more quickly than the Danish (or their French and Austrian allies) had anticipated was offset by the dramatic failure of the German attempts to land on Funen for the first four days of the campaign. The Royal Danish Navy was small but enjoyed clever planners and an arsenal that was designed to defend the archipelago; minelaying had begun on March 10th, the day of mobilization, and the minefields in the Langelands Belt, the Great Belt, and the Fehmarn Belt were not entirely at their full capacity the morning of March 14th, but within two days they would be close to eighty percent of their planned coverage, in addition to minefields laid out on the southern approaches to Copenhagen on the Ostersund passage. Additionally, the Royal Danish Navy had twenty submarines deployed "below the Belts," designed with defensive purposes in mind; short-range, quiet, and heavily-armed due to the low diesel needs thanks to close-in submarine bases. As news arrived that the German Army was punching into the Kolding defenses early on the morning of March 14, a preemptive strike was ordered, with seven Danish subs attacking Apenrade's harbor facilities as an additional six slipped into the Flensburg fjord and fired their torpedoes at the gathered fishing and sailboats that were meant to serve as the primary troop transports to attack Funen; the Danish operation against Apenrade and Flensburg did not destroy all German capabilities, but badly limited them, badly delaying the ability to launch an attack across the Little Belt towards Odense on the first day of the war. German U-boats were deployed through the Kiel Canal ahead of the main battle group intended to clear the Great Belt, with the threat of Danish submarines clearly having been underestimated; the stage, thus, was set for the first major naval engagement of the conflict, and in Germany and Denmark the most famous - the Beltschlact, or the Battle of the Belts..."
- The Central European War
