The rear communications of existing armies are based on railways, the advanced communications on roads, both of which have proved inadequate to stand even the _internal_ strain of modern warfare. In the last war they suffered little _external_ interference from enemy aircraft, but in the future this is a certainty. Both these means of communication depend on fixed tracks, which cannot be varied save after a long period of labour and preparation; since they are shown on the map they are easily located and can be kept under observation from the air. If railways, because of their visibility and limited number of routes, are in themselves the more vulnerable, no more helpless target exists than long columns of slow-moving infantry on the march. A vivid picture of the chaos caused by air attack is to be found in Major-General Gathorne-Hardy’s account of the ghastly fate of the Austrian columns and transport after Vittorio Veneto in October, 1918. If they are not bombed out of existence, air-attack will at least force them to disperse and take cover so frequently as to slow up their rate of advance to a snail’s pace, while days of bombing by hostile aircraft will hardly be a tonic for their moral. Thus the nation which continues to base its military communications on railways and roads is running for a fall. What is the alternative? The opposite method to tracked movement is trackless――by means of caterpillar track or multi-wheeled vehicles capable of quitting the roads at will on the approach of hostile aircraft, and of advancing on a wide front, instead of through a bottleneck. If infantry, because of certain limitations on tank-action, may still survive for a time as a battle-instrument, it is the merest common sense that they should be transported to the battlefield, their 3–5 m.p.h. legs replaced by 15–25 m.p.h. mechanical tracks――not only because they may thus be kept fresh for their fighting _rôle_, but because otherwise they will never reach the battlefield at all. The advent of aircraft has had another important consequence. Just as in the wider sphere, their power to hop over a hostile army enables them to strike direct at the political and industrial centres of the nation, so in the zone of the armies has it laid bare the life-line of the hostile army itself――its communications. The obvious antidote to this new development is to make the communications fluid instead of rigid, and by putting the supply and transport of armies on a trackless basis, we not only revive their “punch” by endowing them with mobility, but extract much of the sting from the military form of the air attack.