Crater warfare came as a development of intensified artillery attacks upon trench systems. It was at Dunajec on the eastern front that for the first time in modern war the wheels of artillery were placed hub to hub in intensified hurricane fire upon enemy positions. The result there under von Mackensen's direction was the rout of the Russians. When later the same tactics were employed on the western front, the result was to destroy whole trench systems with the exception of deep dugouts, and to send the occupants of the trenches into the craters, made by shell explosions, for protection. It was observed that, these craters made excellent cover and when linked by vigorous use of the intrenching tools carried by every soldier, they made a fair substitute for the trenches. This observation gave root to an idea which was followed by both armies; this was the deliberate creation of crater systems by the artillery of the attacking force. Into these lines of craters the attacking infantry threw itself in wave after wave as it rushed toward the enemy trenches. The ground is so riddled by this intensive artillery fire that there is created what is known as "moon terrain", fields resembling the surface of the moon as seen through a powerful telescope. Troops on both sides were trained to utilize these shell holes to the utmost, each little group occupying a crater, keeping in touch with its nearest group and moving steadily in unison toward the enemy. One detail in which this war surpassed all others was in the use of machine guns and grenades. The Germans were first to make extensive use of the machine gun as a weapon with which to produce an effective barrage. They established machine-gun nests at frequent intervals commanding the zone over which infantry was to advance and by skilful crossfire kept that terrain free from every living thing. The Germans preferred a machine gun, water cooled and of the barrel-recoil type. The English used a Vickers-Maxim and a Lewis gun, the latter the invention of an officer in the American army. The French preferred the Hotchkiss and the Saint-Etienne. The Americans standardized the Browning light and heavy machine guns, and these did effective service. It was asserted by American gunnery experts that the Browning excels all other weapons of its type. Two general types of grenades were used on both sides. One a defensive bomb about the size of an orange, containing a bursting charge weighing twenty-two ounces. Then there was a grenade used for offensive work carrying about thirty-two ounces of high explosives. The defensive grenades were of cast iron and so made that they burst into more than a hundred jagged pieces when they exploded. These wounded or killed within a radius of one hundred and fifty yards. In exceptional instances, the range was higher.