 Hitler's plans to invade Norway and Denmark in the spring of 1940 led to the withdrawal of the fleet's surface warships and most of the ocean-going U-boats for fleet operations in Operation Weserübung.The resulting Norwegian campaign revealed serious flaws in the magnetic influence pistol (firing mechanism) of the U-boats' principal weapon, the torpedo.Although the narrow fjords gave U-boats little room for manoeuvre, the concentration of British warships, troopships and supply ships provided countless opportunities for the U-boats to attack.