	Attrition to the enemy was easy to assert and difficult to refute and thus may have been a convenient facesaving excuse in the wake of many indecisive battles. It is, in many cases, hard to see the logic of warfare by attrition because of the obvious uncertainty of the level of damage to the enemy and of the damage that the attacking force may sustain to its own limited and expensive resources while trying to achieve that damage. Historians such as John Terraine and Gary Sheffield have suggested that attritional warfare was, however, a necessary step on the road to eventual victory, a 'wearing down process' that sapped Central Powers strength and left them vulnerable during the Hundred Days campaign of 1918.