the frontier fortresses, and even in the ranks of Napoleon's army, could
conduct their military operations on a very different plan from that
which would be adopted by either Austria, Prussia, Russia, England,
Spain, Portugal, Holland, Italy, and the German powers, if singly waging
war with the French. Napoleon sometimes detached a corps to observe a
fortress which threatened his line of operations or of manoeuvre; at
others, he delayed his advance till the place could be reduced.

"An army," says Jomini, "may sometimes penetrate between places on an
open frontier, to attack the enemy's forces in the field, taking care at
the same time to _observe_ these places; but no invading army can cross
a great river, like the Danube, the Rhine, or the Elbe, without reducing
at least one of the fortresses on that river, so as to secure a line of
retreat; but being in possession of such a place, it can continue the
offensive, while its _matériel de siège_ successively reduces the other
places."

In case the main army is obliged to remain and cover the besieging
corps, it should take some central position, where it can command all
the avenues of approach, and fall with vigor on the enemy, should he
attempt to raise the siege. Napoleon's operations before Mantua, in
1796, offer the finest model for imitation.

The old system of intrenched camps and lines of contravallation is
unsuited to the spirit of modern warfare. In ancient times, and more
particularly in the middle ages, too much importance was attached to
tactical positions, and not enough to strategic points and lines. This
gave to fortifications a character that never properly belonged to them.
From the middle ages down to the period of the French Revolution, wars
were carried on mainly by the system of positions--one party confining