Two new means of action, or instruments of warfare, that became prominent in the first world war were the principal causes of military discussion and controversy during the interval that followed —the aircraft and the tank. In those twenty years they met more doubt and criticism than recognition of their potentialities. Yet when the second world war came they largely dominated its course—especially in the opening stages.

Another new instrument, a naval one, was of earlier origin, but was allowed no adequate chance to prove its powers until the first world war. This was the submarine. Only after the surface duel between the battle-fleets had clearly become barren, by the middle of the war, was the submarine given such a chance—in the hands of the inferior naval power, Germany. But then, in 1917, it became dominant in struggle at sea, and by its potency in blockade brought the superior naval power, Britain, to the verge of defeat by starvation. Yet after that war it soon fell into neglect, and the possibility of a revival of its threat was discounted by the bulk of naval opinion, which clung to the illusion, and faith, that the battleship was again the mistress of the seas—so that when the next great war came in 1939 even Germany had only a handful of submarines. But these soon became an important factor, and, as their numbers increased, a vital one— even though their effect was never quite so great as in the previous war.