Previous to 1914 it was generally believed that torpedo attacks would play a large part in any great naval engagement, and this was the reason why all naval advisers insisted that a large number of these vessels should be constructed as essential units of the fleet. Yet the war had not made much progress when it became apparent that this versatile craft had another great part to play, and that it would once more justify its name in really heroic fashion. Just as it had proved its worth in driving the surface torpedo boat from the seas, so now it developed into a very dangerous foe to the torpedo boat that sailed beneath the waves. Events soon demonstrated that, in all open engagements between submarine and destroyer, the submarine stood very little chance. The reason for this was simply that the submarine had no weapon with which it could successfully resist the attack of the destroyer, whereas the destroyer had several with which it could attack the submarine. The submarine had three or four torpedo tubes, and only one or two guns, and with neither could it afford to risk attacking the more powerfully armed destroyer. The U-boat was of such a fragile nature that it could never afford to engage in a combat in which it stood much chance of getting hit. A destroyer could stand a comparatively severe pounding and still remain fairly intact, but a single shell, striking a submarine, was a very serious matter; even though the vessel did not sink as a result, it was almost inevitable that certain parts of its machinery would be so injured that it would have difficulty in getting into port. It therefore became necessary for the submarine always to play safe, to fight only under conditions in which it had the enemy at such a disadvantage that it ran little risk itself; and this was the reason why it preferred to attack merchant and passenger ships rather than vessels, such as the destroyer, that could energetically defend themselves.