The Norwegian campaign (8 April – 10 June 1940) involved the attempt by Allied forces to defend northern Norway coupled with the Norwegian military's resistance to the country's invasion by Nazi Germany in World War II.

Planned as Operation Wilfred and Plan R 4, while the German attack was feared but had not yet happened, the battlecruiser HMS Renown set out from Scapa Flow for Vestfjorden with twelve destroyers on 4 April. The Royal Navy and the Kriegsmarine met at the first and second naval battles of Narvik on 10 and 13 April, and British forces conducted the Åndalsnes landings on 13 April. The main strategic reason for Germany to invade Norway was to seize the port of Narvik and guarantee the delivery of iron ore needed for German steel production.

The campaign was fought until 10 June 1940 and saw the escape of King Haakon VII and Crown Prince Olav to the United Kingdom. A British, French and Polish expeditionary force of 38,000 troops landed in the north. It had moderate success but made a rapid strategic retreat after the Battle of France began on 14 May. The Norwegian government then went into exile in London. The campaign ended with the occupation of the entirety of Norway by Germany but elements of the Norwegian military escaped and fought on overseas.

Britain and France had signed military assistance treaties with Poland and two days after the German Invasion of Poland on 1 September 1939, both declared war on Nazi Germany. However, neither country mounted significant offensive operations and for several months there were no major engagements, and this period became known as the Phoney War or "Twilight War". Winston Churchill in particular wished to escalate the war into a more active phase, in contrast to Prime Minister Neville Chamberlain.

During this time both sides wished to open secondary fronts. For the Allies, in particular the French, this was based on a desire to avoid repeating the trench warfare of the First World War, which had occurred on the Franco-German border.

Following the outbreak of the Second World War, the Norwegian government had mobilized parts of the Norwegian Army and all but two of the Royal Norwegian Navy's warships. The Norwegian Army Air Service and the Royal Norwegian Navy Air Service were also called up to protect Norwegian neutrality from violations by the warring countries. The first such violations were the sinkings in Norwegian territorial waters of several British ships by German U-boats. In the following months, aircraft from all the belligerents violated Norwegian neutrality.

And almost immediately after the outbreak of war, the British began pressuring the Norwegian government to provide them with the services of the Norwegian merchant navy, being in dire need of shipping to oppose the strength of Nazi forces. Following protracted negotiations between 25 September and 20 November 1939, the Norwegians agreed to charter 150 tankers, as well as other ships with a tonnage of 450,000 gross tons. The Norwegian government's concern for the country's supply lines played an important role in persuading them to accept the agreement.

Norway, although neutral, was considered strategically important for both sides for several reasons. First was the importance of iron ore from Sweden – upon which Germany depended – exported through the Norwegian port of Narvik. This route was especially important in the winter months when much of the Baltic Sea was frozen over. Narvik became of greater significance to the British when it became apparent that Operation Catherine, a plan to gain control of the Baltic Sea, would not be realized. Großadmiral Erich Raeder had pointed out several times in 1939 the danger to Germany of Britain seizing the initiative and launching its own invasion in Scandinavia, for 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.^{[citation needed]}