The phase of military deadlock passed slowly into one of aggression upon the combatant populations behind the fronts by the destruction of food supplies and by attacks through the air. And also there was a steady improvement in the size and range of the guns employed and of such ingenious devices as poison-gas shells and the small mobile forts known as tanks, to break down the resistance of troops in the trenches. The air offensive was the most revolutionary of all the new methods. It carried warfare from two dimensions into three. Hitherto in the history of mankind war had gone on only where the armies marched and met. Now it went on everywhere. First the Zeppelin and then the bombing aeroplane carried war over and past the front to an ever- increasing area of civilian activities beyond. The old distinction maintained in civilized warfare between the civilian and combatant population disappeared. Everyone who grew food, or who sewed a garment, everyone who felled a tree or repaired a house, every railway station and every warehouse was held to be fair game for destruction. The air offensive increased in range and terror with every month in the war. At last great areas of Europe were in a state of siege and subject to nightly raids. Such exposed cities as London and Paris passed sleepless night after sleepless night while the bombs burst, the anti-aircraft guns maintained an intolerable racket, and the fire engines and ambulances rattled headlong through the darkened and deserted streets. The effects upon the minds and health of old people and of young children were particularly distressing and destructive. Pestilence, that old follower of warfare, did not arrive until the very end of the fighting in 1918. For four years medical science staved off any general epidemic; then came a great outbreak of influenza about the world which destroyed many millions of people. Famine also was staved off for some time. By the beginning of 1918 however most of Europe was in a state of mitigated and regulated famine. The production of food throughout the world had fallen very greatly through the calling off of peasant mankind to the fronts, and the distribution of such food as was produced was impeded by the havoc wrought by the submarine, by the rupture of customary routes through the closing of frontiers, and by the disorganization of the transport system of the world. The various governments took possession of the dwindling food supplies, and, with more or less success, rationed their populations. By the fourth year the whole world was suffering from shortages of clothing and housing and of most of the normal gear of life as well as of food. Business and economic life were profoundly disorganized. Every-one was worried, and most people were leading lives of unwonted discomfort.