When, on the 5th of February, 1915, the German admiralty proclaimed a "war zone" around the British Isles and announced that it would fight the sea power of the Allies with submarines, a new era in naval warfare had opened. In all previous wars, and in the earlier months of the Great War, submarines were employed as auxiliaries to the larger naval units. The Germans were the first to use them as separate units. The idea of sending a fleet of submarines out on to the high seas was a new one, and had been impossible in the last war in which they had been used--that between Russia and Japan. But the improvements which had been made in their design and equipment since then had made an actual cruising submarine possible, and made possible the new phase of naval warfare inaugurated by the German admiralty.
While Germany was the last great sea power to adopt the submarine as a weapon, both England and Germany, in the years immediately preceding the war, had spent the same amounts of money on this sort of craft--about $18,000,000--but while the Germans had later given as much attention to them as to any other sort of naval craft, the British authorities did not figure on employing the submarine as a separate offensive tactical unit being sufficiently equipped in large ships carrying large guns. And being weaker in capital ships Germany was compelled to rely upon underwater warfare in her campaign of attrition. Not only were the naval authorities of the rest of the world uninformed about the improvements that German submarines carried, but they were fooled even as to the actual number which Germany had built.
The most modern of the German submarines at the time had a length of 213 feet and a beam of twenty feet, these dimensions giving them sufficient deck space to mount thereon two rapid-fire guns, one of 3.5 inches and another of 1.4 inches. Their displacement was 900 tons, and they could make a speed of 18 knots when traveling "light" (above water), and 12 knots when traveling submerged. These speeds made it possible for them to overtake all but the fastest merchantmen, though not fast enough to run away from destroyers, gunboats, and fast cruisers. Their range of operation was 2,000 miles, and in the early months of 1915, it was possible for Germany to send two or three of them from their base in the North Sea to the Mediterranean. Germany was at the same time experimenting with a larger type, with a displacement of 1,200 tons and an operating distance of 5,000 miles.