The convoy system, as I have already explained, was essentially an offensive measure; it compelled the submarine to encounter its most formidable antagonist, the destroyer, and to risk destruction every time that it attacked merchant vessels. This system, however, was an indirect offensive, or, to use the technical phrase, it was a defensive-offensive. Its great success in protecting merchant shipping, and the indispensable service which it performed to the cause of civilization, I have already described. But the fact remained that there could be no final solution of the submarine problem, barring breaking down the enemy _moral_, until a definite, direct method of attacking these boats had been found. A depth charge, fired from the deck of a destroyer, was a serious matter for the submarine; still the submarine could avoid this deadly weapon at any time by simply concealing its whereabouts when in danger of attack. The destroyer could usually sink the submarine whenever it could get near enough; it was for the under-water boat, however, to decide whether an engagement should take place. That great advantage in warfare, the option of fighting or of running away, always lay with the submarine. Until it was possible for our naval forces to set out to sea, find the enemy that was constantly assailing our commerce, and destroy him, it was useless to maintain that we had discovered the anti-submarine tactics which would drive this pest from the ocean for all time. Though the convoy, the mine-fields, the mystery ships, the airplane, and several other methods of fighting the under-water boat had been developed, the submarine could still utilize that one great quality of invisibility which made any final method of attacking it such a difficult problem.