Over the longer run, the Allies' terrifying experience with the submarine in 1917 enriched the warfare of the future. Submarine development continued among all the main belligerents in the two decades after the war. The potential of that weapon was now evident, and when war broke out again in 1939, the British and German submarines of the day went into action immediately and with crushing effect. The Germans made the biggest headlines. The sinking of the battleship Royal Oak early on the morning of October 14, 1939, when a German U-boat penetrated the defenses at Scapa Flow, was Jellicoe's nightmare of 1914 turned into grim reality. But Berlin made its greatest commitment of underwater forces to snap the "Atlantic Bridge" connecting Britain and the Western Hemisphere in 1942; it was the linear descendant of their earlier effort in 1917. Here Admiral Karl Doenitz, a veteran of World War I, initiated wolf pack tactics in an effort to counter the defensive power of convoys. This technique had been used briefly in May 1918; it involved several U-boats striking simultaneously at the ships of a single convoy. At that time, the tactic failed, partly because British intelligence learned of the danger and rerouted vulnerable vessels. Allied countermeasures also drew on the experience of 1917-1918. The British government applied the same system of food rationing and promotion of agriculture in the second war that they used in the first. And the military countermeasures likewise took up the earlier pattern. Convoys were the norm from the start of the war, and aerial patrols played a key part in harassing and sinking the submarine. The depth charges and location devices, such as sonar, were modem versions of those tools applied or tried in the earlier war. Nonetheless, during the interwar period, navies continued to center their strategy and building programs on the battleship. In a plea to the United States secretary of the navy, the navy's General Board wrote in August 1937: "The battleship is the basic instrument of naval warfare. ... An orderly program of replacement must be instituted ... to prevent our battle line strength from falling to a third rate status." The plea was based on concern that the United States would be left behind as Germany, Japan, France, and Italy were busy building their great ships of the line. It took World War II and the lessons of both the submarine and the aircraft carrier to change this emphasis. For example, when the United States joined the war against Japan in December 1941, both belligerents made the submarine a key weapon in the Pacific. The ability of the United States Navy to cut off naval traffic to and from the home islands of Japan by the late spring of 1945 was the dream of World War I submariners transformed into successful reality. The subsequent history of the submarine, now nuclearpowered, has made it the premier weapon in naval warfare.