When war broke out in Europe on 1 September 1939, none of the belligerents planned for, or expected, Scandinavia to become a theatre of operations. Germany viewed a neutral Norway as in its best interest. When Norway issued a declaration of neutrality on 1 September 1939, the German response came on 2 September. The Germans stated that they would respect Norwegian neutrality as long as that neutrality was maintained in an uncompromising manner. The German view was that if that neutrality was not maintained in such manner, or if a third power violated Norwegian neutrality, Germany would be forced to protect its interests by ways and means dictated by the situation at the time. Germany also gave a clear warning that it would not tolerate a tilt of Norwegian neutrality towards the Western Allies. Britain and France, however, did not issue an immediate response to the Norwegian declaration of neutrality. The British government, however, stated it would respect Norwegian neutrality in case of war but it also considered that a German attack on Norway would be an attack on Great Britain.
    By late 1939, Hitler and the German High Command knew the importance of Norway remaining free of British domination. The control of Norway’s extensive coast line would be very important in the battle for control of the North Sea. This would ease the passage of German warships and submarines into the Atlantic. The control of Norway would also aid Germany’s ability to import iron ore from Sweden. Before the invasion of France, U-boats had to either travel via the Straits of Dover or north of Scotland. Either route was dangerous. A port in northern Norway would have eased this – though by no means would it have ended the problem of getting into the Atlantic. In 1929, Vice-Admiral Wolfgang Wegener had published a book, “The Sea Strategy of the World War,” in which he stated that Germany should seize Norway in a future war so that the German Navy of the future would have an easier time getting to the Atlantic. The commander of the German Navy, Grand Admiral Erich Raeder, did not agree with his theory, but it did attract the attention and support of many other German naval officers.
    At the start of the war, Germany imported, annually, about 10 million tons of iron ore from Sweden. Nine million tons of this came from north Sweden via the port of Luleå. However, this port freezes over for the winter months and the Norwegian port of Narvik does not. Therefore, control of Narvik, in the north of Norway, would have been very important to the Germans in easing the movement of iron ore to Germany.
    Rather than seize Norway, Admiral Raeder wanted to rely on Norway remaining neutral in the war and the Allies respecting this neutrality. The Norwegians also believed that the British Navy would be available to them if the Germans attempted to invade. As late as March 1940, British chiefs-of-staff believed that a German invasion of Norway via the sea would not work. From the Norwegian point of view, a power that wants to land in Norway, whether in the south or in the north must rule the sea, and the power that rules the sea has no need to land in Norway.
    In November 1939, Winston Churchill was appointed First Lord of the Admiralty. He had a fascination for the indirect approach in warfare and for striking in what he perceived to be enemy weaknesses. Norway was considered to be a weak German flank. The mining of the Norwegian port of Narvik and other ports along the coast line of Norway would ultimately halt the shipment of iron ore from Sweden to Germany. But that was not considered sufficient. Maximum pressure was needed to bottle up the German Navy in the North Sea and the Baltic in order to repeat the economic blockade of World War I. This move would have to be coordinated with France and would include French, Polish and British army troops to land in Norway and possibly move into Sweden.
    On 5 February 1940, the British and French military staffs presented their plans to the Allied Supreme War Council in Paris. The War Council, based on these plans, approved a British motion to dispatch a military expeditionary force of several brigades of British, French and Polish troops to take control of Norway and northern Sweden. This operation was code named Avonmouth. The iron ore mines of northern Sweden were also included in the objective of the expeditionary force.
    On 14 February, the destroyer HMS Cossack violated Norwegian territorial waters and seized the Altmark - the former supply ship of the German raider Admiral Graf Spee. This was done to release 299 British merchant seamen that the Altmark was bringing back to Germany as prisoners. This provided further evidence to Hitler that Britain had complete disregard for Norwegian neutrality. Finland was also on the verge of breaking down under the renewed Soviet offensive. This put pressure on Hitler to react to the Scandinavian crisis with utmost speed. With events now unraveling fast, on 21 February 1940, Hitler ordered Lt. General Nicholaus von Falkenhorst to prepare for the invasion of Norway and Denmark. The code name for the invasion was operation Weserubung.
    The operation to invade Norway was in itself contrary to all principles of naval warfare. Against the vastly superior Royal Navy, the German Navy stood no chance in achieving its objectives to land troops in the designated major ports of Norway, keep them supplied and protect them from interference by the Royal Navy. Admiral Raeder, who himself was a pessimistic realist, expressed to Hitler that in a campaign to seize Norway, "...it was possible that Germany might lose her entire fleet."
    In the face of overwhelming British Naval superiority, the Germans gambled most of their fleet. The German plan would have to base its operation on speed, maneuver, deception, and surprise in order to successfully seize Norway. The German plan called for attacking various ports along the Norwegian coast simultaneously. The troops participating in the initial assault would be carried on fast moving warships to avoid the Royal Navy. Transport aircraft like the Ju-52 would be used initially to drop paratroops to seize key areas around the capital, Oslo, and later to transport troops to reinforce the initial landing force. All of this depended on the complete cooperation between the German Navy, the Luftwaffe, and the Army. It was the job of General von Falkenhorst to coordinate the various tasks of the assault groups.
    In the meantime Winston Churchill continued to press the War Cabinet to support his original plan to mine the iron-ore route inside Norwegian territorial waters. He argued that Germany would bring pressure to bear on Norway if the minefields were laid and that this, in turn, would force the Norwegian government to turn to the Allies for assistance. This was typical of Winston Churchill wishful thinking that eventually gained him the Cabinet approval for his scheme.
    The mine laying sortie, code-named Operation Wilfred, was scheduled for 8 April and the ships taking part sailed on the 5th.  Meanwhile, British troops, scheduled for Norway, were embarked at various Scottish bases. There were also French troops including the Foreign Legion, which were moved earlier from France to bases in Britain. A Polish mountain brigade was present as part of the Allied force.
    On 8 April, a Polish submarine, Orzel, sank the German ship SS Rio de Janeiro off the Norwegian coast. Norwegian fishing boats picked up survivors who turned out to be German soldiers – thus confirming the British belief that German troops were being moved by sea on their way to Norway. Actually the entire German invasion force set sail on 3 April.
    The German attack was carried out, on 9 April, with swiftness and surprise. The forces assigned to the invasion of Norway included the Army’s XXI Corps, with 5 infantry and one mountain division, the Luftwaffe with more than 1000 aircraft and several parachute battalions, and the Navy with all its capital ships that were available at the time, all the heavy and light cruisers, and 14 of the Navy’s 20 destroyers. To complement the main force, a large number of torpedo boats, minesweepers, escorts, and auxiliary ships were present. The plan called for the capture of six primary targets by landing troops from warships at: Oslo, Kristiansand, Bergen, Trondheim, Narvik, and Egersund. The attack was designed to quickly overwhelm the Norwegian defenders and occupy these vital areas before any form of organized resistance could be mounted. The Luftwaffe’s job was to monitor British movements and provide support for the ground troops and naval forces. Against Denmark, two infantry divisions were used to capture key cities, and the Luftwaffe landed parachute troops in airfields outside Copenhagen. It was hoped that German troops could avoid armed confrontation with the native populations in both countries, and German troops were instructed only to fire if fired upon. They were also told to tell the local natives, "We come as friends!"
    The first clash with the Allies was between the British destroyer Glowworm and the German heavy cruiser Admiral Hipper. The British destroyer had been detached from the forces escorting the British mine layers on 6 April. Two days later she sighted the German destroyer Bernd von Arnim. She was carrying some of the troops from the invasion force, and so turned to run back to the support of the Hipper. The first salvo from the German cruiser hit the Glowworm. Her captain, Lieutenant-Commander Gerard Broadmead Roope, decided to ram the German cruisers. Fearing a torpedo attack, the Hipper also turned to ram, but too slowly, and she was struck in the side by the Glowworm. The British destroyer then sank with heavy loss of life, while the Hipper took damage that kept her out of action throughout May. For his action, Lieutenant-Commander Roope received posthumously the first Victoria Cross in World War II. It was the only time that the enemy - Hipper's captain, Helmuth Heye - recommended it!
    The only real German setback on 9 April came at Oslo. There the new German heavy cruiser Blücher was sunk by gunfire and torpedoes from the Norwegian fortress at Oscarborg, preventing the naval expedition from reaching the Norwegian capital.
    The first Allied counterattacks were directed against the German navy. On 10 April five British destroyers attacked the ten German destroyers at Narvik. They caught five of the German destroyers by surprise in Narvik harbor, sinking two and damaging the remaining three, before losing two destroyers themselves to a German counterattack. The remaining eight German destroyers and one U-boat were destroyed on 13 April by the battleship HMS Warspite supported by nine destroyers.
    On 10 April also saw a more ominous success for the British. The light cruiser Königsberg was laying vulnerable at Bergen. On the day after the invasion it was attacked by ten Blackburn Skua dive bombers, based on the Orkney Islands and became the first major operational warship to be sunk by hostile aircraft.
    The second part of the German attack began on 12 April when troops struck out in the areas around Oslo. The attack to the east forced the Norwegians up against the Swedish border, where by 15 April 3,000 men had gone into internment. The attack to the west was also successful, and by 16 April the German forces in southern Norway were in contact with each other.
    In Narvik, the small isolated garrison of 2,000 soldiers and 2,500 sailors from the sunken destroyers was attacked in the mountains by the Norwegians, but hardly troubled by the British. Eventually a large French contingent arrived, under General Marie Émile Béthouart. He was far more willing to take risks, and on 13 April the French Foreign Legion took part in the first opposed amphibious landing of the war, capturing Bjerkvik, at the head of Nerjangs Fjord, north of Narvik. By that time the German invasion of France and the Low Countries was well under way, and it was soon decided to withdraw from Narvik. Only now was Narvik attacked. On 28 May the town fell into Allied hands, to be held for just over a week. Before they left, the Allies destroyed the port and the ore handling facilities, at least delaying iron ore shipments to Germany.
    The Germans then launched Operation Juno. This was a naval offensive to interdict Allied transports in the Norwegian Sea. The German ships involved were the Scharnhorst and Gneisenau, the heavy cruiser Admiral Hipper and four destroyers. The mission was launched on 8 June 1940.  On his own initiative, however, the German commander, Admiral Wilhelm Marschall, decided to seek and destroy the Allied transports. The troop transport Orama, the tanker Oil Pioneer and the minesweeper HMS Juniper were sunk. Marschall ordered the Hipper and the destroyers to Trondheim, where they arrived in the morning of 9 June.
    While sailing through the Norwegian Sea, the British aircraft carrier Glorious and her two escorts, the destroyers HMS Acasta and HMS Ardent, were intercepted by the German battlecruisers Scharnhorst and Gneisenau. The carrier and her escorts were sunk in two hours, with the loss of 1,519 men; there were only 45 survivors. The single survivor from Acasta was rescued by the Norwegian merchant ship Borgund which also saved 38 men from one of Glorious' lifeboats. All 39 men saved by Borgund were set ashore in the Faroes Islands on 14 June.
    The Norwegian Campaign ended on 9 June 1940. One can say that the campaign was history’s first combined air, sea, land, and special operations campaign. German leadership realized from the beginning that the margin for success was narrow. The entire effort hinged on the cooperation of the various services to achieve maximum surprise. The losses sustained by the German Navy were severe. All three capital ships were damaged, one heavy and two light cruisers sunk. Ten destroyers, six U-boats, two large torpedo boats, and 15 lighter vessels were sunk. There were also 21 merchant ships sunk. The  lost 127 aircraft other sources claim only 90 aircraft were lost. In terms of casualties, German losses were estimated to be 5,296 killed, wounded and missing.
    The Allies also sustained heavy losses. The Royal Navy gave its losses as, one aircraft carrier, two cruisers, seven destroyers, and four submarines. The French and Poles each lost one destroyer and one submarine. Allied merchant shipping losses were high, 70 ships of all kinds were sunk during the campaign. British casualties were listed as 1,869 killed, wounded, and missing. Also 2,500 British sailors perished at sea. The French and the Poles gave their losses as 533. The Norwegian armed forces claimed 860 killed, 1,700 wounded, and 400 civilians killed.
    Norway would remain under German occupation for the rest of the war. Only Soviet forces would enter Norway in October 1944 in pursuit of the retreating Germans around Kirkins. They remained there to the end of hostilities.
    During World War II, Norway served as a base for the German 20th mountain army that was attacking the Soviet Union in Murmansk area. The German 5th Air Fleet and U-boats operated from northern Norway to attack Allied convoys that were heading to Archangel and Murmansk in the Soviet Union. These convoys carried essential war materiels to aid the Soviet Union in its desperate struggle against Germany. The German battleship Bismarck used Norway for hiding on its first and last sortie into the Atlantic. The battleship Tirpitz, and other cruisers and destroyers, would use fiords in northern Norway to act as ‘a fleet in being’ to threaten Allied convoy traffic going to the Soviet Union.
    The invasion of Norway in 1940 was a great gamble for Germany and particularly the German Navy. The gamble paid off. It kept a good portion of the Royal Navy tied up in the North Sea for the rest of the war.