This is an idea I have seen a few historians mention. For example, Michael Clemmesen, a historian of the Danish army during this period, states that in August 1916 following the Romanian entry into the war: “even the addition of a mobilised Danish Army of around 100,000 to Germany’s enemies might be critical”.

However, this idea is not realistic due to the Denmark’s foreign policy and army at the time. The Danish defence minister, Peter Munch, and the Radical-Liberal Party of which he was a member, were very anti-militaristic. They viewed a strong army as drawing too much attention to Denmark, being futile anyway in the event of conflict with a great power, and also as being immoral. Therefore the Danish army remained weak both before and throughout the war, being undermanned and undersupplied, with little moral support from the Danish government. Whilst the Danish army had been mobilised to 47,000 men in August 1914, by late-1915 this had been reduced to 23,000. In comparison, the neutral Netherlands maintained a 200,000 strong army throughout the war.

Therefore, German plans drafted in 1916 for a pre-emptive invasion of Denmark (‘Fall J’), were not against the threat of Denmark entering the war, but of the Allies using this opportunity to open a northern frontier. The Germans feared two scenarios with regards to Denmark:

- A British military landing on Jutland

- A British naval offensive through the Kattegat into the Baltic

The Allies were aware of Denmark’s useful position with regards to Germany, being described as a “Dardanelles of the North”. Quite a few British military figures had contemplated the idea of bringing Denmark voluntarily or forcible into the war. Prior to 1916, Winston Churchill as First Lord of the Admiralty had advocated for military action in and around Denmark, but this was dismissed because, in the words of Maurice Hankey, there was:

“no reasonable probability that [Denmark] will voluntarily enter the war, and it would be inconsistent with our attitude towards the German violation of Belgium for us to force them to do so."

In October 1916, when the possibility of Denmark entering the war was considered once again, an Admiralty memorandum on ‘The Military Situation in Denmark’ described the Danish army as incapable of taking defensive (let alone offensive) actions against Germany, and that it would be too dangerous to send a British expeditionary force to Denmark until the German fleet had been destroyed. Therefore, according to the historian Patrick Salmon, although the British in 1916 had originally considered offering Denmark assistance in the event of a German attack, they decided to simply reassure Denmark that they did not intend to drag them into the war.

The above may be enough to answer why the Allies did not convince Denmark to join the war. However, it was a false assumption on the part of many military planners that Denmark could be convinced to join the war by being offered Schleswig (and Holstein).

Following Denmark’s defeat in the war with Prussia and Austria in 1864, in which it had received no military assistance from Britain, France, or Russia, Denmark’s whole outlook of itself changed. Denmark recognised that it was a small power within the German sphere of influence. On top of its attitude with regards to the futility of a military defence, Danish politicians near unanimously began favouring a policy of neutrality in all great power disputes.

However, this neutrality was not unbiased. Despite the Danish populations natural and growing hostility towards Germany, Danish neutrality favoured Germany so as to not aggravate their larger neighbour and provoke a German attack on Denmark. Historians therefore describe Danish foreign policy during this period as being “clearly pro-German” or even (perhaps a bit exaggerated) as a “German client state”.

Denmark’s, albeit limited, military preparations during the war were in fact mostly aimed against a British landing, as opposed to a German invasion, as the Danish government knew this would keep Germany satisfied and not feel threatened or exposed, and provoke an attack on Denmark. But the British assumed that most Danish soldiers would not fire on Allied troops in the event of an Allied landing.

Although most people in Allied countries would have viewed Denmark as a friend of the Allies who had a natural grievance with Germany, policy-makers, such as in the British Foreign Office, were aware of Denmark’s German leaning foreign policy and, perhaps surprisingly, were understanding of this. Before the outbreak of the war, Sir Edward Grey, the British Foreign Secretary, had told his Danish counterpart that Britain would never be the first to violate Danish neutrality and would avoid placing Denmark in “an embarrassing position”. British policy towards Denmark was generally lenient, so as to avoid a situation whereby Germany would violate Denmark’s neutrality. During the war, Denmark continued to export large amounts of bacon, eggs, and butter to Britain, and provoking a German occupation of Denmark would mean Germany had access to these foodstuffs, thus limiting the effectiveness of the Allied blockade.

The Danish government of course had an interest in recovering the Danish part of Schleswig. However, firstly, its independence was of primary importance and it would not take drastic actions to threaten this. Secondly, Denmark was hoping its complacent policy towards Germany would eventually lead to the peaceful restoration of northern Schleswig to Denmark. This was not an unrealistic hope, with various German figures hinting at the possibility of Denmark being given this territory so long as it continued its (pro-German) neutrality in the event of a great power conflict.

Therefore in summary:

1. Ignoring Germany’s military overstretch; Denmark was not in a position to, nor did it have the desire to, invade Germany
2. The Allies were aware and had considered this, but Denmark was too weak and geographically vulnerable
3. Denmark, despite its natural desire to regain lost territory, was observing (a rather pro-German) neutrality to protect its independence, which Allied decision-makers understood

Sources:

Clemmensen, Michael H., The Danish Armed Forces 1909-1918: Between Politicians and Strategic Reality (Copenhagen: Royal Danish Defence College, 2007)

Hankey, Maurice, The Supreme Command: 1914-1918 (London: George Allen & Unwin, 1961)

Holbraad, Carsten, Danish Neutrality: A Study in the Foreign Policy of a Small State (Oxford: Clarendon Press, 1991)

Kaarsted, Tage, Great Britain and Denmark 1914-1920 (Odense: Odense University Press, 1979)

Paulin, Christian, ‘German War Plans against Denmark 1916-1918’ in Small Powers in the Age of Total War: 1900-1940, ed by Hermann Amersfoot and Wim Klinkert (Leiden: Brill, 2001), 119-34.