Back of the trench, the guns of smaller caliber which destroy the barbed wire place a "curtain of fire," as it is called, which does not permit the enemy to escape from a trench, or any reserves to come to his assistance. This process is kept up for such a length of time as is deemed sufficient. At a given moment, the invader charges, often protected by a screen of smoke which is sent out from his own trenches. As the burrowers in the earth crawl from the parapets and take to their legs, they know that their fate is almost altogether dependent upon the preparation by the guns rather than any effort of their own. Ahead of them is this wall of smoke and dust from the explosions, in which they are lost to the observer. Keeping units together and protecting them is as difficult as maneuvering ships in a fog. The delicate problem of the gunner is to protect the invader just as far forward as possible, without putting shells into his own men. A few from defective fuses must fall short. This is expected and is a part of the cost of a charge; but none with correct fuses and dependable powder should. The gunners time their part to that of the invader, by lifting their fire from the first to the second line trench, as their own men are entering the first. Granted that the barbed wire is cleared and the men enter the enemy's trench, they may find themselves struggling over heaps of dust mixed with the rags of sandbags, splintered timbers and the flesh and uniforms of their enemy--at first see not a single adversary. They will be instantly due for heavy shell fire; and also for heavy machine gun and rifle fire from the second line enemy trench. They begin to dig at once in order to establish protection. Out of this wreckage they have to reverse the enemy's trench, so that it shall face toward him. This becomes a matter of desperate effort and usually it is in the course of this that the severest casualties are suffered. But should the artillery destruction of the trench be imperfect, upon entering it they may still take the enemy by surprise in his dugouts. In that case, bombs in hand, at the doorways of these cellars they demand surrender. In case it is not given, they throw the bombs into the dugout; for, to enter, means that they will be shot down.