Thus, when General Lee could not be driven out of his works at Fredericksburg by direct attack, General Hooker marched his army up the river, and by crossing there placed himself nearer Richmond than General Lee was. This compelled General Lee to abandon his position at Fredericksburg, and to meet General Hooker in the open field; otherwise there would have been nothing to prevent General Hooker from going to Richmond, with a part of his greatly superior force, leaving the rest of it to check any operations Lee might have undertaken against his communications.
It is in some such fashion as this that every battle is brought about. One side is ever trying to get somewhere, and the other side is ever trying to prevent it from doing so. Incidentally, each army is trying to destroy the other.
When one army has planted itself in a position of its choice, and the other advances to attack it, this is what happens:—
The army that is standing still throws out lines of pickets in front to watch for the enemy’s advance and report it.
The enemy, as he advances, also throws out a cloud of skirmishers to “feel” of the position and avoid traps and ambushes.
A line of battle often extends over several miles in length, covering all available ground for attack or defence.
Before the advancing general can determine against what part of his adversary’s line to hurl his heaviest battalions, he must study the conditions along that line; namely, by means of his skirmishers.
In the same way the general who is awaiting attack tries to discover through his skirmishers what his enemy’s plan of battle is, and at what points he most needs to concentrate his own men.
While awaiting this information he posts his men—cavalry, artillery, and infantry—wherever he thinks they will be most useful, having reference all the time to their rapid movement during the battle from one part of the line to another, as occasion may demand.
He also holds a considerable part of his army “in reserve.” That is to say, he stations it at points a little in rear of the line of battle, from which he can order all or any part of it to any point where strength may be needed.