army A B, finding he cannot cross and drive the enemy from their
works, determines, by a flank march to the left, to go around them,
crossing at the point E. In order to effect this he must send his trains
off by the road I K L to some interior line, and then slowly unfold his
masses upon the single road K E H. By the time the head of his column is
at H the rear has not perhaps left K, and thus the whole length of his
army is exposed on its side to an attack by the enemy, which may sever
it into two unsupporting portions. It will be perceived that to
accomplish such marches with security, they must be made in secret as
far as possible, until a portion of the marching force reaches the rear
of the enemy; the column must be kept compact, and great vigilance must
be exercised. In his progress from the Rapidan to the James, General
Grant made three movements of this character with entire success, each
time putting our forces so far in the rear of the rebels that they were
compelled to hasten their own retreat instead of delaying to avail
themselves of the opportunity for attacking.

Besides the topography of the country, various circumstances influence
the manner in which a march is conducted--particularly the position of
the enemy. When following a retreating foe, the cavalry is sent in the
advance, supported by some infantry and horse artillery, to harass the
rear guard, and, if practicable, delay the retreat until the main army
can come up. This was the case in the peninsula campaign, from Yorktown
to the Chickahominy. Again, the exact position of the enemy may not be
known, or he may have large bodies in different places, so that his
intentions cannot be surmised. It is then necessary to scatter the army
so as to cover a number of threatened points, care being exercised to
have all the different bodies within supporting distances, and to be on
guard against a sudden concentration of the enemy between them. This was
the case in the campaign which ended so gloriously at Gettysburg. The
rebels were then threatening both Harrisburg and Baltimore, and the two
extremities of our army were over thirty miles apart, so as to be
concentrated either on the right, left, or centre, as events might
determine. It happened that a collision was brought on at Gettysburg,
and both armies immediately concentrated there. The corps on the right
of our army was obliged to march about thirty-two miles, performing the
distance in about eighteen