Trenches did a great job of protecting the troops while they were in them. But neither side was content to simply let their soldiers hunker down indefinitely and simply wait for the other side to attack. The Germans and the Allies all launched attacks against the enemy trenches in an effort to break through the enemy lines and resume a war of movement and deep penetration into the enemy’s territory. In practice, however, most attacks failed to make any significant break throughs or gain territory; the attackers were mowed down by entrenched machine guns, and entangled on barbed wire.
The photo above poignantly illustrated the murderous nature of trench warfare. In this photo you can see a German trench which has been cleared of its German defenders but at such great cost to the attacking French soldiers, that the trench has become a mass grave for the French. The corpse filled ditch has been filled in and covered over to serve as a mass grave, and a stark reminder of the bloody futility of the war on the western front.