In the wake of the Battle of the Marne—during which Allied troops halted the steady German push through Belgium and France that had proceeded over the first month of World War I—a conflict both sides had expected to be short and decisive turns longer and bloodier, as Allied and German forces begin digging the first trenches on the Western Front on September 15, 1914.

The trench system on the Western Front in World War I—fixed from the winter of 1914 to the spring of 1918—eventually stretched from the North Sea coast of Belgium southward through France, with a bulge outwards to contain the much-contested Ypres salient. Running in front of such French towns as Soissons, Reims, Verdun, St. Mihiel and Nancy, the system finally reached its southernmost point in Alsace, at the Swiss border. In total the trenches built during World War I, laid end-to-end, would stretch some 35,000 miles—12,000 of those miles occupied by the Allies, and the rest by the Central Powers.

As historian Paul Fussell describes it, there were usually three lines of trenches: a front-line trench located 50 yards to a mile from its enemy counterpart, guarded by tangled lines of barbed wire; a support trench line several hundred yards back; and a reserve line several hundred yards behind that. A well-built trench did not run straight for any distance, as that would invite the danger of enfilade, or sweeping fire, along a long stretch of the line; instead it zigzagged every few yards. There were three different types of trenches: firing trenches, lined on the side facing the enemy by steps where defending soldiers would stand to fire machine guns and throw grenades at the advancing offense; communication trenches; and “saps,” shallower positions that extended into no-man’s-land and afforded spots for observation posts, grenade-throwing and machine gun-firing.

While war in the trenches during World War I is described in horrific, apocalyptic terms—the mud, the stench of rotting bodies, the enormous rats—the reality was that the trench system protected the soldiers to a large extent from the worst effects of modern firepower, used for the first time during that conflict. The greatest danger came during the periods when the war became more mobile, when the soldiers on either side left the trenches to go on the offensive. German losses per month peaked when they went on the attack: in 1914 in Belgium and France, 1915 on the Eastern Front, and 1918 again in the west; for the French, casualties peaked in September 1914, when they risked everything to halt the German advance at the Marne. Trench warfare redefined battle in the modern age, making artillery into the key weapon. Thus the fundamental challenge on both sides of the line became how to produce enough munitions, keep the troops supplied with these munitions and expend enough of them during an offensive to sufficiently damage the enemy lines before beginning an infantry advance.