Control of Norway was crucially important to Germany's ability to use its sea power effectively against the Allies, particularly Britain. While Norway was at peace, and unoccupied by either of the fighting powers, there was no threat. But the weakness of Norwegian coastal defences, and the inability of her field army to resist effectively a determined invasion by a stronger power were clear. Admiral Raeder had pointed out several times in 1939 the potential danger to Germany of Britain seizing the initiative and launching its own invasion in Scandinavia - if the powerful Royal Navy had bases at Bergen, Narvik and Trondheim, the North Sea would be virtually closed to Germany, and the Kriegsmarine would be at risk even in the Baltic.

At the end of 1939, there had been negotiations between Raeder and Vidkun Quisling, the head of the Norwegian Najonal Samling (National Unity) Party, and a plan had been hatched for a takeover of Norway by political bloodless revolution, following which Quisling would have asked Germany for "protection". But, because Quisling, whose name has become synonymous with "traitor", had only around 10,000 Norwegians behind him, that idea was abandoned and, on January 27th, Hitler ordered detailed contingency planning of "Operation Weser", the taking of Norway. At that stage, there was no commitment to the attack on Norway that was to follow in the spring, but two events moved Hitler to the point of action in April.

The first was the USSR's second, and successful assault on Finland, which threatened to put Russia within striking distance both of Swedish mineral resources and of the Norwegian ports. The second, which finally committed Hitler to action, was the interception by HMS Cossack , a British destroyer commanded by Captain Philip Vian, of the German ship Altmark , officially the supply ship to the Graf Spee , which was carrying 299 British Merchant Navy prisoners back to Germany. When the Norwegians had searched the vessel at Bergen, they had not found the prisoners, who were battened down below decks. But, on the direct orders of Winston Churchill, Captain Vian informed the Norwegians that he was stopping the Altmark , boarded the prison ship in Jossing Fjord on February 16th, and released the prisoners, bringing them back to Britain. This made it clear to all concerned that the Norwegians were powerless to act when either power breached international law in Norwegian waters, and emphasised to Hitler the validity of Raeder's fears about a British occupation of Norway. The result was the issuing of two Fuehrer Directives, on February 26th and March 1st, which both appointed a commander for the Norwegian operation, General Nikolaus von Falkenhorst, and made clear that Denmark was to be occupied as part of the same operation.

The invasion of Norway, originally set for March 20th, but postponed to April 9th, was the first major test for the Kriegsmarine . The plan required five major naval groups to take part, respectively taking Narvik, Trondheim, Bergen, Kristiansand/Arendal, and Oslo. Most of the Kriegsmarine's available warships were actively involved, and the five groups sailed at various times between the 6th and the 8th of April so as to arrive off their targets simultaneously. Two army corps went with them, the 21st for Norway and the 31st for Denmark. Over a thousand German aircraft were committed to the assault. Amazingly, the Norwegian Government had been provided via a friendly Dutch military attache on April 4th with advance warning of the attack, but had not acted upon the information - if they had mobilised in time, history might have been somewhat different.

The assaults on Trondheim and Bergen went well for the Germans, aside from the damaging by shellfire (and subsequent sinking by the British Fleet Air Arm) of the light cruiser Konigsberg at Bergen. The Karlsruhe was torpedoed and sunk at Kristiansand by a British submarine (the Truant ). At Stavanger, the paratroops took Sola airfield and the town without difficulty.

Narvik was taken quickly, but not without incident, and largely by treachery. Commodore Paul Bonte's German destroyers demanded the surrender of two Norwegian coastal defence vessels, the Eidsvold and the Norge . When their commander refused to surrender, his ships were blown out of the water. Almost immediately, Lieutenant-General Dietl led his 3rd Gebirgsjager Division into the town, and received its surrender from a Quisling, Colonel Konrad Sundlo.

The assault on Oslo was not so easy. The pocket battleship Lutzow was badly damaged by the Norwegian shore batteries in the fjord, and the Blucher was sunk by shore-launched torpedoes with the loss of 1,600 lives before the 163rd division managed to capture a large part of Oslo, the arsenal at Horten and the airfield. But for this daring enterprise by Major General Engelbrecht, the battle would have lasted much longer, for it was the ability to land paratroops, in Ju52 aircraft on that airfield, late on April 9th, that determined the course of events in favour of Germany.

Although Norwegian intentions were good, and the willingness of King Haakon, his government and country to fight was evident, Norway's organisation was less than effective and rendered the effort expended on defence largely valueless. General mobilisation was announced only after the invasion had begun. The Commander in Chief was unfit for service, and had to be replaced after the invasion on April 11th by Colonel Otto Ruge, who was hastily promoted to General. The command structure was hopelessly inadequate to deal with a situation as desperately demanding of strong military action. As the evacuation of 24,000 allied troops from Narvik and Harstad was completed two months later, King Haakon and the Norwegian Government were to escape narrowly from Tromso to England aboard HMS Devonshire on June 8th and Vidkun Quisling was to proclaim himself the head of the Norwegian Government, saying "I shall be called the big traitor". In that at least he was right.

Denmark caused the Germans less difficulty than Norway. King Christian X and his government yielded to the German ultimatum, although there was Danish military resistance at Jutland, and the Royal Guard made a stand at the Amalienborg Palace. The Luftwaffe strafed Vaerlose Airfield, headquarters of the Danish Air Force, destroying many aircraft, but there was little reply. The Danish Navy offered no resistance at all. Denmark seemed to have accepted her fate. Of the 15,450 Danes under arms, only 13 were killed and 23 wounded.

As the German invasion of Norway began, the British were not standing idly by, but the Allies' reaction was appallingly late, disastrously ineffective and disorganised. Admiral Sir Charles Forbes had sailed from Scapa Flow on the evening of the 7th, three days after news of the invasion had been leaked to the Allies. His force of two battleships, one battle-cruiser, four cruisers and 21 destroyers headed for Norway to provide cover for minelaying operations, but with orders that suggested the threatened invasion might be a blind for a break-out of the blockaded German fleet to the Atlantic. On April 8th, one of the destroyers, HMS Glowworm , met the Hipper and was severely damaged. With tremendous courage, her captain used his doomed ship to ram the Hipper, radioing as he did so that the German Fleet was at sea.

As a result, the British Admiralty did exactly what the Germans hoped it would do by instructing Admiral Forbes not to follow the German fleet into Trondheim, which would have been tactically sound, but to intercept the Scharnhorst and Gneisenau , which were operating as an independent battle squadron, and were in fact functioning as bait to keep the Royal Navy away from the landings in Norway. By the evening of April 9th, Germany was in control of the most important strategic positions in Norway and Denmark, and Hitler had achieved his easiest bridgehead yet. In Britain, Prime Minister Neville Chamberlain, who only days earlier had announced that Hitler had missed the bus, was in deep trouble.

Britain now joined battle in earnest. On April 10th, the First Battle of Narvik brought five British destroyers into conflict with 10 German destroyers, and cost each side two destroyers and one senior commander. The Royal Navy also sank eight German merchant ships and the ammunition carrier Rauenfels . On the 11th, the RAF attacked Sola airfield, Stavanger, and lost one Wellington out of six. On the 13th came the Second Battle of Narvik, and a significant victory for the Royal Navy. HMS Warspite and nine destroyers attacked eight German destroyers and sank or disabled all of them. By the 14th, the British North Western Expeditionary Force had begun to land at Harstad, the 24th Guards Brigade arriving on the island of Hinnoy, near Harstad, some 96km from Narvik, and separated from the port by a sea channel and snow-covered mountains, on the 15th. On the 16th, the 146th Infantry Brigade landed at Namsos, and on the 17th the 148th landed at Andalsnes. The Germans were now isolated in Narvik, although with plenty of captured Norwegian weapons in addition to their own, and with a "Mountain Marine" unit of 2,600 men, survivors of the annihilated destroyer fleet, to use the Norwegian rifles and machine guns. This force was ordered to "hold out for as long as possible". Each day, the RAF attacked German military installations, and the sea war continued with losses on both sides.

The traitor Quisling, meanwhile, had been ousted on April 15th, and replaced by an "Administrative Council" of Norwegian bureaucrats and lawyers. Suddenly and belatedly, this government, on April 18th, declared war on Germany. On the 19th, the British 146th Infantry Brigade, advancing South from Namsos, reached Verdal, 80km from Trondheim, and the French Chasseurs Alpins - fully trained mountain troops - landed at Namsos. The plan was that these forces would advance on Trondheim and link up with the Norwegian forces retreating northwards, but the arrival of substantial reinforcements for General von Falkenhorst's army, despite the attempt at a blockade of German shipping by Allied submarines in the Skaggerak, made the Allies look again at the weaknesses of their position.

The British troops in Norway were not properly equipped or trained for the position in which they found themselves. Because of the poor organisation of their embarkation, they had not even their full complement of stores and equipment. The German army was well-organised, well-trained and recently reinforced. It quickly became clear that the British plan to retake the centre of Norway could not work and, as if to emphasise the point, on April 26th the German 196th Division, on the right of the 21st Corps, succeeded in joining up with the 181st Division on the left, south of Trondheim.

All Allied efforts were now concentrated on Narvik, and the blocking of the vital route for iron resources through the port. British and French troops withdrew from Andalsnes and Namsos, King Haakon and General Ruge (and the Norwegian gold reserves) headed for Tromso, and the final phase of the Norwegian battle began on April 26th. It lasted effectively until May 28th, when the French 13th Foreign Legion Demi-Brigade finally recaptured Narvik, although by then other events in the Low Countries and France had captured the news. The Germans expected the Allies to try to make their stay in Narvik permanent - but on June 7th the French and British departed, having covered the evacuation of the Allied armies from Norway. Norway and Denmark were totally in German hands.