The successes in the east enabled the Germans to move back to the offensive in the west. The early trenches dug in 1914 had by now become elaborate systems of defense. Both lines of trenches were protected by barbed-wire entanglements 3 to 5 feet high and 30 yards wide, concrete machine-gun nests, and mortar batteries, supported farther back by heavy artillery. Troops lived in holes in the ground, separated from the enemy by a no-man’s land. The unexpected development of trench warfare baffled military leaders who had been trained to fight wars of movement and maneuver. Taking advantage of the recent American invention of the Caterpillar tractor, the British introduced tanks on the Western Front in 1915, but their effectiveness in breaking through enemy defenses was not demonstrated. The only plan generals could devise was to attempt a breakthrough by throwing masses of men against enemy lines that had first been battered by artillery barrages. Periodically, the high command on either side would order an offensive that would begin with an artillery barrage to flatten the enemy’s barbed wire and leave the enemy in a state of shock. After “softening up” the enemy in this fashion, a mass of soldiers would climb out of their trenches with fixed bayonets and hope to work their way toward the opposing trenches. The attacks rarely worked, as the machine gun put hordes of men advancing unprotected across open fields at a severe disadvantage. In 1916 and 1917, millions of young men were sacrificed in the search for the elusive breakthrough. In ten months at Verdun, 700,000 men lost their lives over a few miles of terrain. Warfare in the trenches of the Western Front produced unimaginable horrors. Battlefields were hellish landscapes of barbed wire, shell holes, mud, and injured and dying men. The introduction of poison gas in 1915 produced new forms of injuries, but the first aerial battles were a rare sideshow and gave no hint of the horrors to come with air warfare in the future. Soldiers in the trenches also lived with the persistent presence of death. Since combat went on for months, soldiers had to carry on in the midst of countless bodies of dead men or the remains of men dismembered by artillery barrages. Many soldiers remembered the stench of decomposing bodies and the swarms of rats that grew fat in the trenches.