Surprise is the soul of war. The submarine illustrates this elemental principle, and its astounding development is the most amazing fact of the World Struggle. Given favourable circumstances it can attack when least expected, pounce on its prey at such time as may be most convenient to itself, and return to its lair without so much as being sighted. What has become a vital means to the most important military ends was once described by the British Admiralty as “the weapon of the weaker Power.” To a large extent, of course, it is _par excellence_ the type of vessel necessary to bidders for Sea Supremacy who would wrest maritime predominance from a stronger Power. On the other hand, it has rendered yeoman service to the British Navy, as many of the following pages will show. Germany, a nation of copyists but also of improvers, diverted the submersible from the path of virtue which previous to the outbreak of hostilities it was expected to pursue. It is safe to say that few people in Great Britain entertained the suspicion that underwater craft would be used by any belligerent for the purpose of piracy. Up to August 1914 the submarine was intimately associated in the public mind with death and disaster—death for the crew and disaster for the vessel. It is so easy to forget that Science claims martyrs and Progress exacts sacrifice. These are two of the certainties of an uncertain world. The early stages of aviation also were notable for the wreck of hopes, machines, and men. To-day aircraft share with submarines and tanks the honour of having altered the aspect of war. The motor-car, once the laughing-stock of everybody other than the enthusiast, and now grown into a Juggernaut mounting powerful guns, is the foster-father of the three, for the perfection of the internal combustion engine alone made the submarine and the aeroplane practicable. For good or for ill, the underwater boat has passed from the experimental to the practical. In the hands of the Germans it became a particularly sinister and formidable weapon. The truth is not in us if we attempt to disguise the fact. When there was not so much as a cloud the size of a man’s hand on the European sky, and the Betrayer was pursuing the path of peaceful penetration all undisturbed and almost unsuspected, the submarine was regarded by many eminent authorities as a somewhat precocious weakling in the naval nursery. They refused to believe that it would grow up. Even Mr H. G. Wells, who has loosed so many lucky shafts, unhesitatingly damned it in his _Anticipations_. He saw few possibilities in the craft, and virtually limited its use to narrow waterways and harbours.