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 Compiègne, 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 Neufchâtel-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 Compiègne.
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 along its entire distance.