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.