Secondly, the advance was prepared for by the use of big guns in enormous quantities and in new ways. The number of guns brought into use in this offensive far exceeded that put into the Verdun offensive of 1916, which had been looked upon as the extreme of possible concentration of artillery. The shell fire was now to be directed not only against the trenches, but also far to the rear of the Allied positions. This would break up roads, railways, and bridges for many miles behind the trenches and prevent the sending of reinforcements up to the front. Vast numbers of large shells containing poisonous "mustard" gas were collected. These were to be fired from heavy guns and made to explode far behind the Allied lines. By this means suffocation might be spread among the reserves, among motor drivers, and even among the army mules, and by deranging the transport service make it impossible to concentrate troops to withstand the German advance. In the third place, "shock" troops composed of selected men from all divisions of the army, were to advance after the bombardment, in a series of "waves." When the first wave had reached the limit of its strength and endurance, it was to be followed up by a second mass of fresh troops, and this by a third, and so on until the Allies' defense was completely broken. By their excess in numbers and by these newly devised methods of warfare the German leaders hoped to accomplish three things: (1) to separate the British army from the French army; (2) to seize the Channel ports and interrupt by submarines and big guns the transportation of men and supplies from England to France; and (3) to capture Paris and compel the French to withdraw from the war. Let us now see how and why the Germans failed to secure any one of these three objectives, and how the Allied forces resumed the offensive in the summer of 1918.