On 20 October 1914, SM U-17 sank the first merchant ship, the SS Glitra, off Norway.[8] Surface commerce raiders were proving to be ineffective, and on 4 February 1915, the Kaiser assented to the declaration of a war zone in the waters around the British Isles. This was cited as a retaliation for British minefields and shipping blockades. Under the instructions given to U-boat captains, they could sink merchant ships, even potentially neutral ones, without warning.