<P> Pioneered by the Princess Patricia's Canadian Light Infantry in February 1915, trench raids were carried out in order to capture prisoners and "booty"--letters and other documents to provide intelligence about the unit occupying the opposing trenches . As the war progressed, raiding became part of the general British policy, the intention being to maintain the fighting spirit of the troops and to deny no man's land to the Germans . As well, they were intended to compel the enemy to reinforce, which exposed his troops to artillery fire . </P> <P> Such dominance was achieved at a high cost when the enemy replied with his own artillery, and a post-war British analysis concluded the benefits were probably not worth the cost . Early in the war, surprise raids would be mounted, particularly by the Canadians, but increased vigilance made achieving surprise difficult as the war progressed . By 1916, raids were carefully planned exercises in combined arms and involved close co-operation of infantry and artillery . </P> <P> A raid would begin with an intense artillery bombardment designed to drive off or kill the front - trench garrison and cut the barbed wire . Then the bombardment would shift to form a "box barrage", or cordon, around a section of the front line to prevent a counter-attack intercepting the raid . However, the bombardment also had the effect of notifying the enemy of the location of the planned attack, thus allowing reinforcements to be called in from wider sectors . </P> <P> The intensity of World War I trench warfare meant about 10% of all fighting soldiers were killed . This compared to 5% killed during the Second Boer War and 4.5% killed during World War II . For British and Dominion troops serving on the Western Front, the proportion of troops killed was 12.5%, while the total proportion of troops who became casualties (killed or wounded) was 56% . </P>

World war 1 attacks often began with a of cannon fire