One of the key factors that contributed to the heavy casualties sustained by the British at the Battle of the Somme was the perceived belief that shrapnel would be effective at cutting the barbed wire entanglements in no man's land (although it has been suggested that the reason for the use of shrapnel as a wire-cutter at the Somme was because Britain lacked the capacity to manufacture enough HE shell). This perception was reinforced by the successful deployment of shrapnel shells against Germany's barbed wire entanglements in the 1915 Battle of Neuve Chapelle, but the Germans thickened their barbed wire strands after that battle. As a result, shrapnel was later only effective in killing enemy personnel; even if the conditions were correct, with the angle of descent being flat to maximise the number of bullets going through the entanglements, the probability of a shrapnel ball hitting a thin line of barbed wire and successfully cutting it was extremely low. The bullets also had limited destructive effect and were stopped by sandbags, so troops behind protection or in bunkers were generally safe. Additionally, steel helmets, including both the German Stahlhelm and the British Brodie helmet, could resist Shrapnel bullets and protect the wearer from head injury: