The Norwegian campaign lasted from april till june 1940 and ended with a withdrawal of allied forces after the invasion of France, leaving the country to the Germans. But what if that could be prevented and Norway managed to stand?
So for the challenge, how could the allies win in Norway? I'm thinking something like the allies taking Narvik and the RAF bombing norwegian railways preventing the Germans from advancing northwards too fast, the high mountains and fiords providing a good natural defence. France would still fall, but the allies in Norway stand and fight, pushing southwards, the "Battle of Britain" taking place over the skies of Norway. The Royal Navy completely blocks the North Sea, maybe even manages to distrupt the supply lines going from the Reich to Norway, eventually leading to the allies making it south to Oslo.
How would allied presence in northern Norway affect the battle of France and the battle of Britain? Would Hitler still attack Yugoslavia/Egypt/USSR if he wasn't "really done" with the British in Europe as OTL? Could we see a hypothetical D-Day in Denmark taking place from Norway if it doesn't fall?