World War I marked a step change because of the development of artillery techniques and the protection provided by trenches. By late 1915, the British Expeditionary Force realised that the effects of artillery fire could not smash an opening in German trench lines or reliably destroy enemy artillery at critical times. They therefore developed artillery techniques to suppress the enemy in trenches to allow their infantry to approach them and to suppress the enemy artillery at critical stages to protect attacking infantry. Thereafter, suppression became the defining British artillery tactic, although it had been first used in the Boer Wars. A moving barrage could suppress a line of front providing covering fire for an attack several miles wide. Shrapnel was the usual ammunition used by the British Army in its barrages. Suppressive fire was used against enemy artillery that attacked the assaulting troops with indirect fire.