During World War I the western front ran through the Aisne department. The Chemin des Dames is a crest line between the Aisne and Ailette valleys. Its location and underground quarries meant that it was a veritable fortress. After the First Battle of the Marne in September 1914, the Germans retreated to the heights above the Chemin des Dames and French troops were unable to flush them out. Then began an endless and deadly confrontation: a war of position. The Chemin des Dames became the scene of great battles. Today it is an open-air museum, extending 40 km from east to west, with many monuments along its length.