Many of Napoleon's brilliant victories resulted from merely bringing
troops to bear suddenly upon some decisive point. Rivoli in 1796-7,
Marengo in 1800, Ulm in 1805, Jena in 1806, Ratisbon in 1809, Brienne in
1814, and Ligny in 1815, are familiar examples. But this concentration
of forces, even with a regular army, cannot be calculated on by the
general with any degree of certainty, unless his communications are
perfectly secure. And this difficulty is very much increased where the
troops are new and undisciplined. When a country like ours is invaded,
large numbers of such troops must suddenly be called into the field. Not
knowing the designs of the invaders, much time will be lost in marches
and countermarches; and if there be no safe places of resort the
operations must be indecisive and insecure.

To a defensive army fortifications are valuable as points of repose,
upon which the troops, if beaten, may fall back, and shelter their sick
and wounded, collect their scattered forces, repair their materials, and
draw together a new supply of stores and provisions; and as rallying
points, where new troops may be assembled with safety, and the army, in
a few days, be prepared to again meet the enemy in the open field.
Without these defences, undisciplined and inexperienced armies, when
once routed, can seldom be rallied again, except with great losses. But
when supported by forts, they can select their opportunity for fighting,
and offer or refuse battle according to the probability of success; and,
having a safe place of retreat, they are far less influenced by fear in
the actual conflict.

The enemy, on the other hand, being compelled either to besiege or
_observe_ these works, his army will be separated from its magazines,
its strength and efficiency diminished by detachments, and his whole
force exposed to the horrors of partisan warfare. It has therefore been