I have made this point before, and I now repeat the explanation to emphasize that the patrol system was necessarily unsuccessful, because it made almost impossible any combats with submarines and afforded very little protection to shipping. The advantage of the convoy system, as its advocates now urged, was precisely that it made such combats inevitable. In other words, it meant offensive warfare. It was proposed to surround each convoy with a protecting screen of destroyers in precisely the same way that the battle fleet was protected. Thus we should compel any submarine which was planning to torpedo a convoyed ship to do so only in waters that were infested with destroyers. In order to get into position to discharge its missile the submarine would have to creep up close to the rim that marked the circle of these destroyers. Just as soon as the torpedo started on its course and the tell-tale wake appeared on the surface the protecting ships would immediately begin sowing the waters with their depth charges. Thus in the future the Germans would be compelled to fight for every ship which they should attempt to sink, instead of sinking them conveniently in waters that were free of destroyers, as had hitherto been their privilege. Already the British had demonstrated that such a screen of destroyers could protect merchant ships as well as war vessels. They were making this fact clear every day in the successful transportation of troops and supplies across the Channel. In this region they had established an immune zone, which was constantly patrolled by destroyers and other anti-submarine craft, and through these the merchant fleets were constantly passing with complete safety. The proposal to convoy all merchant ships was a proposal to apply this same system on a much broader scale. If we should arrange our ships in compact convoys and protect them with destroyers we would really create another immune zone of this kind, and this would be different from the one established across the Channel only in that it would be a movable one. In this way we should establish about a square mile of the surface of the ocean in which submarines could not operate without great danger, and then we could move that square mile along until port was reached.