 Attrition was time-consuming so the duration of World War I battles often stretched to weeks and months.The problem with attrition was that the use of fortified defenses in depth generally required a ratio of ten attackers to one defender, or a level of artillery support which was simply not feasible until late 1917, for any reasonable chance of victory.The ability of the defender to move troops using interior lines prevented the possibility of fully exploiting any breakthrough with the level of technology then attainable.