 In May , the 4th Australian , 11th and 24th divisions were moved north from Arras , to become reserve divisions for those corps in the Second Army which were preparing to attack Messines Ridge . Seventy @-@ two of the new Mark IV tanks also arrived in May and were hidden south @-@ west of Ypres . British aircraft began to move north from the Arras front , the total rising to about 300 operational aircraft in II Brigade in the Second Army area . The mass of artillery to be used in the attack was supported by many artillery observation and photographic reconnaissance aircraft , in corps squadrons which had been increased from twelve to eighteen aircraft . Strict enforcement of wireless procedures allowed a reduction of the minimum distance between observation aircraft from 1 @,@ 000 yards ( 910 m ) at Arras in April 1917 to 400 yards ( 370 m ) at Messines , without mutual wireless interference . Wire @-@ cutting began on 21 May and an extra two days were added to the bombardment for more counter @-@ battery fire . The main bombardment began on 31 May , with only one day of poor weather before the attack . Two flights of each observation squadron concentrated on counter @-@ battery observation and one became a " bombardment flight " , working with particular artillery ' " bombardment groups " for wire @-@ cutting and trench @-@ destruction ; these flights became " contact @-@ patrol flights " intended to observe the positions of British troops once the assault began . The attack barrage was rehearsed on 3 June to allow British air observers to plot masked German batteries , which mainly remained hidden but many minor flaws in the British barrage were reported . A repeat performance on 5 June induced a larger number of hidden German batteries to reveal themselves . 
