In preparation for the allied summer offensive in Flanders, the Messines salient had to be eliminated. For years, tunnellers had dug mines beneath German trenches in the area and packed them with explosives. Before dawn on 7 June 1917, 19 mines were blown, obliterating the German positions. The explosions, heard across the English Channel, were the largest planned detonations until the advent of the atomic bomb. It is thought that 10,000 Germans were killed in the initial blasts.
British, New Zealand and Australian troops advanced to take the devastated ridge. Regarded a triumph, Messines also marked the first major battle for the 3rd Australian Division under Major General John Monash. Two of his men, Private John Carroll and Captain Robert Grieve, won the Victoria Cross during the fighting. Despite the success, the battle cost 6,000 Australian casualties.