The Navy's most critical struggle was the Battle of the Atlantic defending Britain's vital commercial supply lines against U-boat attack. A traditional convoy system was instituted from the start of the war, but German submarine tactics, based on group attacks by "wolf-packs", were much more effective than in the previous war, and the threat remained serious for well over three years. Defences were strengthened by deployment of purpose-built escorts, of escort carriers, of long-range patrol aircraft, improved anti-submarine weapons and sensors, and by the deciphering of German signals by the code-breakers of Bletchley Park. The threat was at last effectively broken by devastating losses inflicted on the U-boats in the spring of 1943. Intense convoy battles of a different sort, against combined air, surface and submarine threats, were fought off enemy-controlled coasts in the Arctic, where Britain ran supply convoys through to Russia, and in the Mediterranean, where the struggle focused on Convoys to Malta.