The resumption of ruthless submarine warfare became a serious threat to the cause of the Allies, and at the time of our entry into the war their situation was critical—how much more critical than the world was allowed to know at the time, Admiral Sims has disclosed in his “The Victory at Sea.” The relief brought about through the convoy system, in which our destroyers, the navy’s first participants, had a large share, was immediate and important. But the submarine menace was far from ended and—according to the best information—would soon be greatly augmented, while the increasing number of transports would offer the enemy more opportunities, with the added horror of troopship sinkings in prospect. Of further measures, the most effective would be such a blockade as would keep the submarines in or from their bases. The British had already mined a large area north and west from Heligoland, but this obstruction was not insurmountable, for the Germans from nearby could always clear a passage through when wanted. In any case, until the Skagerrack passage were closed, the submarines might use that route without hindrance. Two weighty reasons kept the Skagerrack open—unwillingness to violate neutral waters, and the ease with which German forces could raid any barrier near their bases. It may be recalled with what sudden damage a small German cruiser detachment raided a convoy just outside the Skagerrack, in October, 1917. To be effective, therefore, any barrier must be beyond easy reach of a raiding force and cover the Skagerrack, and must also be far enough to the northward of the British bases not to hamper the battle fleet’s engaging with the enemy. Hence, the anti-submarine barriers should be, one near Dover Strait, the other across the North Sea, from Scotland to Norway. The closing of Dover Strait, undertaken by the British Navy alone, needs no further mention here. Although the strong tidal currents there, frequent rough seas, and hard, smooth bottom were unfavorable for minefields, other means—such as a line of guard vessels moored not far apart and equipped with powerful searchlights, together with numerous active patrollers—were employed with a considerable degree of success. The Northern Barrage would be too long a front, and much of it too far from base, for effective patrol without a great number of vessels. A wide, thickly sown minefield, however, would watch night and day in all weathers without relief, and would be even more effective against passage submerged than against passage on the surface, because of the less wear and disturbance of the mines by wave action, deep down under water.