So easy did the march to Paris seem, that the Germans grew careless in their advance and then suddenly they met with a reverse that sent them back in full retreat. However, the military authorities of Germany had studied not only how to attack but also how to retreat and how to stand on the defensive. In this, as in every other phase of the conflict, they were far in advance of the rest of the world, and after their defeat in the First Battle of the Marne, they retired to a strong position and hastily prepared to stand on the defensive. When the Allies tried to drive them farther back, they found that the German army had simply sunk into the ground. The war of manoeuver had given way to trench warfare, which lasted through long, tedious months nearly to the end of the great conflict. The Germans found it necessary to make the stand because the Russians were putting up such a strong fight on Germany's eastern frontier. Men had to be withdrawn from the western front to stem the Russian tide, which meant that the western armies of the kaiser had to cease their offensive activities for the time being. The delay was fatal to the Germans, for they had opposed to them not only brave men but intelligent men who were quick to learn. And when the Germans were ready to resume operations in the West, they found that the Allies also had sunk into the ground and had learned all their tricks of trench warfare, adding a number of new ones of their own. The whole character of the war was changed. The opposing forces were dead-locked and neither could break through the other's lines. The idea of digging into the ground did not originate with this war, but never before had it been carried out on so extensive a scale. The inventive faculties of both sides were vainly exercised to find some way of breaking the dead-lock. Hundreds of new inventions were developed. The history of war from the days of the ancient Romans up to the present time was searched for some means of breaking down the opposing lines. However, the dead-lock was not broken until a special machine had been invented, a traveling fort. But the story of that machine is told in another chapter.