World War II/Fall of Denmark and Norway

Northern Europe in German and British strategic considerations
One of the few books that Adolf Hitler read on the topic of naval strategy was The Sea Strategy of the World War (1929) by Wolfgang Wegener, a former Vice Admiral of the German Imperial Navy of World War I. In it, the author confidently asserted that the German navy, which in the world war had proven an uninspired besieged force under constant naval blockade, could have drastically improved its own position vis-a-vis its British rival if only the German Empire had moved swiftly in the opening phases of the war to seize neutral Norway, whose ports would have exposed the northern flank of British naval defenses. Nonetheless, the German military buildup was decisively bipolar rather than tripolar; the navy was left underprioritized compared to the army and air force, whose combined operations were understood by German military and political planners as decisive in any land-based conflict. The German naval commander-in-chief, Erich Raeder, continuously attempted to gain Hitler's attention and support for a more decisive naval expansion program, such as when he held a presentation on his "Basic Thoughts about the Conduct of Naval Warfare" on 2 February 1937.

The potentially exposed position of Northern Europe to a renewed British naval thrust worried German naval and economic planners, particularly with a view towards iron ore imports from Sweden. The General Plenipotentiary for the Economy's office calculated German iron ore imports in the year 1937 to have been ~20,620,000 tons, of which ~9,076,000 tons were produced in Sweden. Of this Swedish production, ~4,889,000 tons were transported from the iron ore mines in northern Sweden westwards across the Norwegian border and then shipped to Germany from the Norwegian seaport of Narvik. The iron ore transport ships thus travelled along the Norwegian coast, only protected by the diplomatic entanglements of Norwegian neutrality from a potential British attack in case of a war between Germany and the United Kingdom.

British and German plans against Norway
With the outbreak of war in September 1939, the Western Allies began to calculate avenues of attack against Germany. In a draft paper by the British chiefs of staff on 4 September, the Chamberlain cabinet was warned that, although unlikely, a preemptive German occupation of Norway was a distinct possibility. In the minds of the British planners, the Germans could execute such a move to prevent any British intervention in Norway that could cut German iron ore supplies.

One of the British ministers particularly enthusiastic about experimental military action was Winston Churchill. Not yet the prime minister, Churchill served under Chamberlain as the First Lord of the Admiralty, i.e. as the civilian minister for naval affairs. As early as 19 September 1939, Churchill suggested with some urgency that neutral Norway's territorial waters should be aggressively mined by the Royal Navy to force German ships out of Norway's neutrality zones, where they could more freely be attacked by the Royal Navy. This move was at this point rejected by the British cabinet, which feared the political ramifications that could be caused among neutral nations if Britain were to start mining neutral waters. Churchill later went on to win approval for a less internationally divisive scheme when he presented on 19 November a plan for a mine barrier between Norwegian waters and the Orkney islands, as had already happened once before during World War I.

On 10 October 1939, German naval chief Raeder once again broached the topic of northern European security with Hitler; in a presentation to the dictator, Raeder stressed the critical dependency of Germany upon Swedish iron ore exports through Norwegian waters and also pitched the idea that Norwegian seaports might be useful as submarine bases for interdiction warfare against British naval imports. At this point in time, a quick victory over France seemed unlikely, which made Norwegian ports an appealing alternative to French ports in the support of German submarine warfare. Raeder was personally motivated by the rivalry inside the German armed forces; whereas army and air force received high degrees of attention, naval officers often felt neglected by the German political leadership. As a result, Raeder hoped to gain political influence and additional financial resources for his naval forces if Hitler were to decide on action against Norway.

The importance of Northern Europe was drastically increased with the outbreak of the Winter War between the Soviet Union and Finland on 30 November 1939. Meanwhile, the Germans were aggressively courted by a potential collaborator in Norway. The Norwegian career soldier Vidkun Quisling, leader of a Nazi-inspired party in Norway, the Nasjonal Samling, met with Raeder on 11 December, having done so on the suggestion to Raeder by Nazi chief ideologue Alfred Rosenberg which Rosenberg made to Raeder after another of the latter's attempts to convince Hitler of the necessity of preemptive German operations in Norway. Quisling, offering himself as the leader of a German-aligned government for the case of a German invasion of Norway, warned that a British preemptive attack against Norway might be imminent. Although unimpressed with Quisling as a person and potential leader of a German-allied Norway, Raeder was nonetheless alarmed at Quisling's suggestion that the Norwegian government might choose to not resist a British landing in Norway, thus effectively joining the Allies without a fight and denying Germany's iron ore supply.

Quisling subsequently had two meetings with Hitler on 14 and 18 December 1939, again discussing the potential British threat against Norway.

At about the same time, Churchill continued pushing the British cabinet towards naval action towards Norway. He reiterated his support for the mining of Norwegian waters on 16 December, and argued to the cabinet on 18 December that the United Kingdom could only gain advantages from the involvement of Norway and Sweden in the war. On 2 January 1940, Churchill pressed his case that British naval action in Norwegian waters might provoke a full-scale German invasion of the country, which would force Norway into the war on the Allied side and thus benefit Britain. Finally, on 6 January, the War Cabinet relented to Churchill's prodding and issued orders to senior naval commanders to assure British combat readiness for a Norwegian mission. However, the War Cabinet rejected on 12 January Churchill's own preferred plan of action towards Narvik, leading to lengthy discussions of a potential three-pronged attack on 19 January 1940, with "Operation Avonmouth" aimed at Narvik, "Operation Stafford" at Trondheim, Bergen and Stavanger, and "Operation Plymouth" at Trondheim, with the latter designed to bring sufficient forces to the Norwegian mainland to potentially involve Sweden in the war as well. The large associated resource allotment – 80,000 men, 10,000 vehicles, and a 60-day landing operation involving 12 passenger liners and 39 store ships, caused severe scepticism among the ranks of the War Cabinet, however.

Just one day after the War Cabinet's discussion of the Avonmouth–Stafford–Plymouth trifecta, the German armed forces' high command (OKW) finished its own preliminary draft for an invasion of Norway, simply titled Studie Nord, "Study North".

Political pressure on the Allied leaders remained high after the unsanctimonious disaster on the Polish front and after the less than satisfying follow-up to the public outcry over the Soviet invasion of Finland, which was still ongoing due to the remarkable resilience of the Finnish defenders. On 5 February 1940, the Allied Supreme War Council decided during a Paris meeting to solve both the iron ore problem and the PR disaster with a joint strike against Norway and Sweden. British and French troops were to land on Norway's coasts, then to march into Sweden to seize the Swedish iron ore mines, and to eventually open supply routes to northern Finland so that the Western Allied could bring volunteer units to reinforce Helsinki's tenuous position.

On 16 February 1940, the "Altmark Incident" accelerated German preparations for an attack against Norway. The German tanker Altmark, which carried 299 British prisoners of war, was attacked and forced to surrender by HMS Cossack inside Norwegian waters. Hitler now fixated on the idea that Norwegian neutrality, which was obviously not respected by the Royal Navy, had become useless to Germany as a political assurance. He subsequently ordered the acceleration for the German operation,now dubbed Weserübung, "Weser Exercise", on 19 February and agreed with OKW chief Wilhelm Keitel on army general Nikolaus von Falkenhorst as a viable candidate to lead the operation. Falkenhorst's experience as an operations officer in the Baltic theater of World War I was an important argument in his nomination. Falkenhorst was summoned to the Reich Chancellery, where he was informed by Hitler on 20 February about Weser Exercise. On Hitler's orders, he quickly worked out a preliminary operational plan based on a tourist guide he bought in Berlin, and presented it later the same afternoon to Hitler's approval.

Falkenhorst took command of his staff, the subsequent XXI Army Corps, on 26 February, and presented a more elaborate plan to Hitler on the 29th. A critical innovation made by Falkenhorst was the occupation of northern Denmark, which had so far played only a minor role in the various considerations by German and British planners. Hitler approved this amendment, and promptly expanded it to include the entirety of Denmark. The dictator also definitively decreed that Vidkun Quisling was to play no role whatsoever in the invasion – even though Quisling would reappear later.

Hitler, who guided the German war effort through several key directives over the course of the war, issued one such Leader Directive (Führerweisung), numbered 11, on 1 March 1940. The plan did not yet have a specific timetable, but placed the air force contingents under Falkenhorst's command. This would go on to outrage the commander-in-chief of the German air force (Luftwaffe), Hermann Göring, who during a conference on the 5th of March managed to change Hitler's mind, thus placing the air force back under his command. Falkenhorst was subsequently forced to visit Göring at his personal estate ("Karinhall") on the 7th to informally discuss the German air force's dispositions for Norway.

The mutual distrust between the two sides peaked in March 1940; German naval high command warned of a British concentration of forces at Scapa Flow in preparation for an invasion of Norway on 8 March, which led to an all-out military emergency in Germany on 11 March. The imminence of this threat was however decreased when the Winter War ended with a negotiated settlement and a conditional Finnish defeat on 12 March. In spite of this strategic setback, Churchill continued to argue in the War Cabinet meeting of 14 March 1940 that the Royal Navy should undertake mining operations in Norwegian waters. Pressure on the British government was increased by the French ambassador, who delivered a frustrated letter from Prime Minister Daladier – later forced to resign due to the resentment of the French public – urging military action against the German iron ore supply routes through the maritime territories of Norway. Though under increasing pressure in the House of Commons, Neville Chamberlain nevertheless held his own and swayed the cabinet away from an invasion of Norway. Instead, the British undertook "Operation Royal Marine", hoping to cause damage to Germany by letting naval mines flow down the Rhine river from the French border into the German industrial heartland.

On the German side, Raeder was anxious to finally get Weser Exercise on the road, arguing in a 26 March discussion with Hitler that a simultaneous operation against Norway and France was not feasible for the German armed forces. He pointed to the lunar cycle as a critical precondition for visibility during night operations and named new moon on 7 April as his preferred date for an invasion, and outright rejecting any date later than 15 April.

On 28 March, the Supreme Allied War Council met and decided on a 'political horse trade', exchanging the British "Operation Royal Marine", the mining of the Rhine, for the French-desired "Operation Wilfred", the mining of Norwegian waters. This agreement was mainly designed to give the new French prime minister Paul Reynaud an important domestic political victory, and paid little attention to German schemes or reactions. While British naval chiefs began to inquire about naval ranges and French troops availabilities, the Germans prepared for final conferences to prepare the invasion.

Hitler discussed Weser Exercise with all key commanders in a final conference on 1 April. On the following day, having received confirmations of operational readiness by all three branches of the armed forces, he placed the invasion date on 9 April 1940 (which was also dubbed "Weser Day") and the operational start at 04:15 in the morning (dubbed "Weser Hour"). Weser Exercise was also definitively separated into two sub-operations, with Weser Exercise North (Weserübung Nord) being directed against Norway, whereas the forces of Weser Exercise South (Weserübung Süd) were to attack Denmark.

Although Chamberlain famously taunted in public that 'Hitler had missed the bus' on 5 April, the Royal Navy was now hurrying forces to undertake its mining operations in time. Though British minelaying began on 05:00 in the morning of 8 April, eleven German naval warship groups had already left their ports on the 7th, and were headed to their rendezvous points. Just after midnight on 8/9 April, the invasion troop ships departed Wilhelmshaven and Cuxhaven, en route for Norway and Denmark.