What made the U-boat seem so formidable in World War I was principally the blindness and obtuseness of the British Admirality. In the run-up to the war, it refused to accept the possibility of a submarine guerre de course and made no real preparations for one.

A close analysis of U-boat successes shows that they sank the overwhelming majority of Allied ships not by torpedo but by deck gun in British coastal waters and in the Mediterranean Sea where marine traffic was dense. Most of these deck guns in the early years were 88mm (3.4"). Inasmuch as the U-boat was seldom an efficient or stable gun platform and the hull was extremely vulnerable to counterfire, had the Admirality promptly armed British merchant ships with slightly more powerful 4" guns manned by trained gun crews, only the bravest of the U-boat skippers would have sought a one-on-one gun contest, and Allied merchant ship losses doubtlessly would have diminished significantly. Several merchant ships so armed sailing in concert would have rendered a U-boat attack by deck gun virtually suicidal, forcing the Germans to attack submerged with scarce, virtually handmade torpedoes from relatively stationary positions, which were easy to evade or outrun.

The most grievous British sin, of course, was the failure to promptly adopt large-scale convoying. By the time ocean convoying was fully in place, September 1917, U-boats had already sunk about 8 million of the total 12 million tons bagged in the war.

The Germans were also blind and obtuse. On the strategic level, the U-boat campaign was the chief factor in bringing the United States into the war, assuring the ultimate defeat of the Central Powers. Moreover, the Germans made the mistake of launching unrestricted submarine warfare before they had anywhere near sufficient U-boats to carry it off. This resulted in an undesirable piecemeal commitment of naval power, which the Allies were able to whittle down bit by bit. On the tactical level, the Germans failed to develop promptly any anticonvoy doctrine, such as group (or “wolf pack”) night surface attacks, massing force against force at the decisive point.

Importantly, the German high command relied completely on the U-boat to interdict the flow of fresh American troops from the States to French Atlantic ports. The U-boats utterly failed in this task. In a quite awesome naval triumph which is usually overlooked, Allied maritime forces transported about 2 million soldiers from the States to France, with the loss of merely fifty-six men due to U-boats.

The reality of the German U-boat campaign in World War I is that it failed. It caused much damage and hardship and created no little terror. However, contrary to the mythology, the campaign did not really come close to bringing Great Britain to her knees, thereby precipitating an Allied defat.

— Clay Blair, Hitler’s U-Boat War: The Hunters, 1939 - 1942