To realize this we have only to ask the question: How can the respective arms protect themselves against gas? Aircraft, by rising above it; tanks, by being air-tight and producing their own oxygen inside; infantry, cavalry, artillery, by the use of some form of respirator. A respirator is only proof against known kinds of gas; it cannot be worn for long without incapacitating its wearer from active exertion; it cannot protect the whole body, unless it be developed into a complete diver’s suit, in which movement would be almost impossible. If a man cannot move freely, he cannot fight. If a horse cannot move, what use is his rider? If the artillery-man cannot serve the gun freely and the gun is immovable, field artillery is useless. Therefore, if gas becomes a standard weapon, we are left with the tank and the aeroplane as the sole effective arms for offensive action. Only as the static defenders of the fortified bases――the land-ports――of tanks and aircraft will there be a future for infantry and artillery, the former armed with super-heavy armour-piercing machine-guns, and the latter with anti-tank and anti-aircraft guns. How long even tanks will persist is a moot point. To hit so small and rapidly moving a target is not easy for the aeroplane, and if it come low, the tank can hit back. In the next lap of the immemorial race between the means of offence and protection, mobility is on the side of the aeroplane, but gravity on that of the tank――in increasing the degree of armour. Again, though gas is the weapon which will sign the death-warrant of the traditional arms, and by which the new arms will attack the enemy nation, its very triumph will cause one more revolution of the eternal cycle. Since both are gas-proof, the armour-piercing projectile will come back into its own for air and tank battles. Both machines also are self-contained fighting organisms, combining hitting power, mobility and protection. What present type of weapon already possesses this combination? The warship. Thus the tactics of tank _versus_ tank will conform to those of naval war, while overhead Tennyson’s “Airy navies grappling in the central blue” find literal and not only figurative fulfilment. Although overland warfare will ultimately assume a close resemblance to sea fighting, the novelists’ dream of land “dreadnoughts” is unlikely of fruition. The obstacles met with on land, the benefit of using an already cleared and graduated path, such as road systems provide through and over these obstacles, the load-capacity and width of bridges, will limit the size of the landships. Even the amphibious tank does not solve the problem of getting out of a river with steep banks.