These sappers are of the utmost importance, for without them an entire
column might be checked and thrown into confusion by impediments which a
few sappers with their axes would remove in a very short time.
Detachments of engineer troops must also act in concert with the cavalry
and artillery for the same purpose as above. In establishing the
batteries of artillery, in opening roads for their manoeuvres, and in
arranging material obstacles for their defence, the axes, picks, and
shovels of the sappers are of infinite value. Fieldworks, bridges, and
bridge-defences, frequently have a decisive influence upon the result of
a battle, but as these are usually arranged previous to the action, they
will be discussed in another place. In the attack and defence of these
field-works, the engineer troops play a distinguished part. The
consideration of this part of the subject, though perhaps properly
belonging to the tactics of battles, will also be postponed to another
occasion.

We will now discuss the employment of the combined arms on the field of
battle.

Before the French Revolution, all the infantry, formed by regiments and
brigades, was united in a single body and drawn up in two lines. The
cavalry was placed on the two flanks, and the artillery distributed
along the entire line. In moving by wings, they formed four columns, two
of cavalry and two of infantry: in moving by a flank, they formed only
two very long columns; the cavalry, however, sometimes formed a third
and separate column in flank movements, but this disposition was rarely
made.

The French Revolution introduced the system of grand divisions composed
of the four arms combined; each division moved separately and
independently of the other. In the wars of the Empire, Napoleon united
two or more of these divisions into a _corps d'armee,_ which formed a
wing, the centre, or reserve of his grand army. In addition to these
divisions and _corps d'armee,_ he had large reserves of cavalry and
artillery, which were employed as distinct and separate arms.

If the forces be sufficiently numerous to fight by _corps d'armee,_ each
corps should have its own reserve, independent of the general reserve of
the army. Again, if the forces be so small as to act by grand divisions
only, each division should then have _its_ separate reserve.

An army, whether composed of separate corps or of grand divisions,
usually forms, on the field of battle, a centre, two wings, and a
r