The German ship Lutzow was badly damaged by the British submarine HMS Spearfish, as a result Lutzow returned to base on April 18th 1940.
Why was Britain and France involved in Norway? The reason was its neighbour - Sweden. In winter, iron ore from Sweden was exported from the Norwegian port of Narvik - whoever occupied Norway would control the supply of iron ore to Germany. If Britain controlled the Norwegian ports, the North Sea would be virtually closed to the German navy, and the Baltic would be in striking distance.