The terrifying North Atlantic supply convoys to Russia began in September 1941 with Churchill's promise to Stalin to send convoys every 10 days.  In the event, treacherous conditions, insufficient ships, limited unloading facilities, and frozen ports, meant that by the year end only seven convoys had got through.  The German Navy were quick to react, C in C Grand Admiral Raeder increasing U-boat strength in Norwegian waters, and ordering the mighty battleship Tirpitz to Norway, prompting Churchill to say "Tirpitz is the most important naval vessel in the situation today".  He believed her destruction would "profoundly affect the course of the war".  The first RAF attack was mounted on the night of 29/30 January, but the great battleship escaped unscathed.  Fearing for Tirpitz's safety, Hitler ordered more Luftwaffe aircraft to Norway, and the gathering German forces began inflicting mounting losses on the lumbering convoys.  As the Arctic spring and summer progressed, bringing perpetual daylight to the Barents Sea, the attacks continued around the clock.  The threat of attack by the Tirpitz was sufficient in itself to cause chaos and disruption to the North Atlantic convoys: The day after convoy PQ17 set sail from Iceland on June 27, the British Home Fleet learned that Tirpitz had sailed from her hideout in the Norwegian fjords and, considering the threat to their cruiser squadron too serious, and with their battleships and carriers unable to arrive in time, PQ17 was ordered to scatter.  U-boats and air attack took a terrible toll, only 10 of the 34 merchantmen having set sail made it through to the Russian port of Archangel.  Tirpitz failed to make contact with the convoy and returned to port without firing a shell, but her very presence in the theatre was enough to cause the demise of convoy PQ17.