he consideration
of the topography of the battles that centered round the crossing
of the Ourcq, Grand Morin, Petit Morin, and the Marne, so the Aisne
is naturally the most important determinant in the problems of
its crossing.

The River Aisne rises in the Argonne, southwest of Verdun. Through
the Champagne region its banks are of gradual slope, but shortly
after it passes Rethel, on its westerly course, the configuration
changes sharply, and at Craonne the bluffs overlooking the river
are 450 feet high. It is easy to see what an inaccessible barrier
is made by such a line of cliffs. For forty miles this line of
bluffs continues, almost reaching to Compiegne, where the Aisne
enters the Oise. Not only are the banks of the Aisne thus guarded
by steep bluffs, but the character of those bluffs is peculiarly
fitted for military purposes. For long stretches along the north
side the cliffs stand sheer and have spurs that dip down sharply
to the valley. The ridge, or the top of the bluff, which looks
from below like the scarp of a great plateau, lies at an average
of a mile or more from the stream. Many of these spurs jut out in
such a way that if fortified they could enfilade up and downstream.
To add to the military value of such a barrier the edge of the scarp
is heavily wooded, while the lower slopes are steep and grassy,
with small woods at irregular intervals. Even from the high ground
on the south bank of the stream, the top of the plateau on the
north cannot be seen, and from below it is effectually cloaked.

Two tributaries are to be considered in this river valley which thus
forms so natural a post of defense. Both flow in from the south,
the Suippe, which joins the main stream at Neufchatel-sur-Aisne and
the Vesle, on which stands the ancient city of Rheims. This river
joins the Aisne a little over seven miles east of Soissons, which
is itself twenty miles east of Compiegne.

The line taken by the German armies for their stand was not the
river itself, but the northern ridge. At no place more than a mile
and a half from the river, it was always within gunfire of any
crossing. Every place of crossing was commanded by a spur. Every
road on the north bank was in their hands, every road on the south
bank curved upward so as to be a fair mark for their artillery.
As the German drive advanced, a huge body of sappers and miners
had been left behind to fortify this Aisne line, and the system
developed was much the same