 On 8 May , the British preliminary bombardment began and on 23 May became much heavier . The breastworks of the front defences were demolished and concrete shelters on both sides of the ridge were systematically destroyed . Air superiority allowed the British artillery observation aircraft to cruise over the German defences , despite the defensive efforts of the Richthofen Circus . On 26 May , the German front garrisons were ordered to move forward 50 yards ( 46 m ) into shell @-@ holes in no @-@ man 's @-@ land at dawn and return to their shelters at night . When the shelters were destroyed , shell @-@ hole positions were made permanent as were those of the companies further back . Troops in the Höhen ( second ) line were withdrawn behind the ridge and by the end of May , the front battalions were being changed every two days instead of every five , due to the effect of the British bombardment . Some German troops on the ridge were convinced of the mine danger and their morale was depressed further by the statement of a prisoner taken on 6 June , that the attack would be synchronised with mine explosions . On 1 June , the British bombardment became more intense and nearly every German defensive position on the forward slope was obliterated . The Luftstreitkräfte effort reached its maximum from 4 – 5 June , when German aircraft observed 74 counter @-@ battery shoots and wireless interception by the British showed 62 German aircraft , escorted by up to seven fighters each , directing artillery fire against the Second Army . British air observation on the reverse slope was less effective than in the foreground but Mesen and Wijtschate villages were demolished , as were much of the Höhen and Sehnen lines , although many pill boxes survived . Long @-@ range fire on Comines , Warneton , Wervicq and villages , road junctions , railways and bridges caused much damage and a number of ammunition dumps were destroyed . 
