An experienced aviator--always there is that word "experience" which has no substitute--might detect a machine-gun nest if he flew low; but not as a rule in woods or in bushes, or even in the open when covered with green branches. There were many machine-gunners and relatively few aviators. If a gunner thought that an aviator who flew low had seen him, he might have taken up a new position before the aviator's information had brought down artillery fire. The machine-gunner was a will-of-the-wisp with a hornet's sting, which could be thrown a mile and a half. Usually the price of locating him was casualties to the infantry, and still more casualties before he was taken, if he stood his ground. If the Germans had not enough machine-guns back of their first line for a complete interlocking defense on the first day of the battle--and they certainly had later--they aimed to place them where they could do the most good. Naturally the American Army, studying its chessboard, had taken into consideration the counter-moves of the enemy which would result from its attack. Of course the passage through the entanglements would lead to the first dislocations of _liaison_; the storming of the trenches to more; and the passage over the shell-craters to still more. After every offensive against the trench system, officers had studied how to avoid the slowing down of the attack after the first line was taken. This had led to passing the first wave promptly through the trenches and leaving a second wave to "mop up" by "breaching" dugouts and cleaning up points of resistance; and then to the system of "leap-frogging," in which, when the men in front had been weakened in numbers by casualties and lost their aggressive cohesion, fresh troops went through them to carry forward the attack. Reserves in passing through the lanes of the barbed wire and over the trenches and on to catch up with an advancing line also suffered from disorganization, which might be increased by strong concentrations of enemy shell and machine-gun fire. A division commander had discretion as to how he would gain his objectives, which brings us into the field of tactical direction, as technical as it is vital to success. His dispositions were a test of his knowledge of his profession, and his handling of the division after it was engaged of his qualities of generalship. In some instances villages and strong points were passed by the main line of advance, and left to be conquered by special attacking forces. Instructions had not only to be elaborate but practical.