Let us first examine the change in the conditions of human life that has altered war from a normal aspect of the conflict for existence of human societies into a terror and a threat for the entire species. The change is essentially a change in the amount of power available for human purposes, and more particularly in the amount of material power that can be controlled by one individual. Human society up to a couple of centuries ago was essentially a man-power and horse-power system. There was in addition a certain limited use of water power and wind power, but that was not on a scale to affect the general truth of the proposition. The first intimation of the great change began seven centuries ago with the appearance of explosives. In the thirteenth century the Mongols made a very effective military use of the Chinese discovery of gunpowder. They conquered most of the known world, and their introduction of a low-grade explosive in warfare rapidly destroyed the immunity of castles and walled cities, abolished knighthood, and utterly wrecked and devastated the irrigation system of Mesopotamia, which had been a populous and civilized region since before the beginnings of history. But the restricted metallurgical knowledge of the time set definite limits to the size and range of cannon. It was only with the nineteenth century that the large scale production of cast steel and the growth of chemical knowledge made the military use of a variety of explosives practicable. The systematic extension of human power began in the eighteenth century with the utilization of steam and coal. That opened a crescendo of invention and discovery which thrust rapidly increasing quantities of material energy into men's hands. Even now that crescendo may not have reached its climax.