against attack. Enormous concrete dugouts, shelters, machine-gun positions were built. Tunnels, often miles long, were dug underground, so that troops might pass quickly from one point in the line to another without being exposed to shell-fire. Acres of barbed wire, strung on steel posts, and twisted about in every direction, were put up before the several lines of trenches. No effort, no cost was spared, to make this system of defenses the strongest in the world. All during the winter the Germans worked at it, and in March, 1917, when everything was ready, von Hindenburg swiftly withdrew his men from their advanced positions before the British lines to their new positions in the rear.

This, however, was not all that he did. When he drew back his armies to their new lines, he issued orders that the country over which they passed should be reduced to an absolute waste. The German troops carried out their orders to the letter. When they retired nothing was left. Every house, village, town was burnt or blown up. Roads were destroyed, bridges shattered, wells contaminated, trees of every sort, even the fruit trees in the French orchards, cut down; nothing was spared. When the German armies finally retired behind the Hindenburg Line the country before them for many miles was a desert. Von Hindenburg knew that it would require weeks, even months, for the French and British to con

struct roads, railways, bridges over this desolate waste. All their preparations for attacking his former lines were now rendered useless. Their huge stores of ammunition, their great guns, their miles and miles of light narrow-gage railroads, running everywhere behind the front to bring up ammunition and supplies, were all now of no value. Everything would have to be moved up, over a country almost impassable. By the time this could be done a large part of the spring and summer would be gone, and winter, with its early rains, would be at hand. It was a shrewd move, and one which enabled the Germans to prolong the war for many months. They knew, from their secret agents, that trouble was coming in Russia, and reasoned that if they could hold back the British and French during 1917, they could attack with greater forces later on. These forces they expected to bring from the Russian front.

The Hindenburg Line, they believed, could never be taken. To drive them out of France by the costly frontal attacks used by the British in the Battle of the Somme would, the Germans argued, require millions of men. Their experts said that before this could be accomplished the French and British armies would be exhausted. A new element, however, was entering the problem of trench warfare.

This was a form of armored automobile, known as the "tank." The reason for this name is a

curious one. The British had earlier in the war imported from America a large number of heavy tractors—that is, traction engines driven by gasolene, and used to haul guns and other heavy loads up to the front. These American tractors were what are known as "caterpillar" tractors, so called because their driving-wheels, instead of resting on the ground, ran on endless steel tracks or belts, so that these wheels were continually laying for themselves a broad track upon which they could pass over soft or rough ground, the track unrolling itself before the wheels, and being picked up after they had passed by the moving belt to which it was attached.

The value of these tractors in passing over the rough and broken ground between the trenches was at once recognized by the British, and they arranged to use them as engines of war. Their plan was to cover the body of the tractors with heavy armor, which should be proof against machine-gun fire, place machine guns inside them, and use them in attacking the enemy machine-gun positions or "nests" which had caused their advancing forces such great losses. Two additional wheels in the form of a trailer were added, to enable the machines to cross shell-holes and trenches, these trailers giving the tractors a much longer wheel base.

The construction of a large number of these engines of war was begun, and in order to pre

serve the utmost secrecy they were referred to, in the official records of the British War Department, as "tanks." The name was merely a blind, to conceal what was being done, but when the machines finally appeared on the battle-field, it clung to them, and they have been known as tanks ever since. They proved, before the end of the war, to be the final solution of the problem of successfully attacking trenches defended by machine-gun fire. When they first appeared on the front, at Courcelette, in the autumn of 1916, the Germans were absolutely terrified. The lumbering monsters, weighing many tons-rolling irresistibly forward over shell-holes and trenches, crushing down wire entanglements, spitting a hail of bullets-caused them to drop their rifles and flee in disorder. Only a well-directed shot from a field gun could put a tank out of action, and such moving targets, in the smoke and dust of battle, were very hard to hit.

Later on, both the French and the British built large numbers of these tanks. Some, of the heavy type, carried crews of six or eight men and were armed with small cannon for destroying opposing tanks or fortifications. Others, of a much lighter type, carried only two men, and were used along with infantry in attacking machine-gun positions, on account of their superior speed. These lighter tanks were known as "whippets."

The influence of the tanks, during the final

stages of the war, can scarcely be overestimated. The Germans built a few of them, but they were heavy and clumsy in design, and accomplished little of value. It was inevitable that the tanks should be of greater service to the Allies, who were on the offensive, than to the Germans, who were on the defensive. They were the final answer to the German machine gun.