 The Second Army had five corps , of which three conducted the attack and two remained on the northern flank , not engaged in the main operation ; the XIV Corps was available in General Headquarters reserve ( GHQ reserve ) . The 4th Army divisions of Gruppe Wijtschate ( Group Wytschaete ) held the ridge , which were later reinforced by a division from Gruppe Ypern . The battle began with the detonation of a series of mines beneath German lines , which created 19 large craters and devastated the German front line defences . This was followed by a creeping barrage 700 yards ( 640 m ) deep , covering the British troops as they secured the ridge , with support from tanks , cavalry patrols and aircraft . The effectiveness of the British mines , barrages and bombardments was improved by advances in artillery survey , flash @-@ spotting and centralised control of artillery from the Second Army headquarters . British attacks from 8 – 14 June advanced the front line beyond the former German Sehnen ( Oosttaverne ) line . The Battle of Messines was a prelude to the much larger Third Battle of Ypres campaign , the preliminary bombardment for which began on 11 July 1917 . 
