On June 7, 1917, the British 2nd Army had a crushing victory over the Germans at Messines Ridge in Belgian West Flanders, marking the successful prelude to an Allied offensive designed to break the grinding stalemate on the Western Front in WW1I.
British forces put careful planning into the Battle of Messines Ridge: for the previous 18 months, soldiers had worked to place nearly 1 million pounds of explosives in tunnels under the German positions. The tunnels extended to some 2,000 feet in length, and some were as much as 100 feet below the surface of the ridge, where the Germans had long since been entrenched.
At 3:10 a.m. on June 7, 1917, a series of simultaneous explosions rocked the area; the blast was heard as far away as London. As a German observer described the explosions: “nineteen gigantic roses with carmine petals, or? enormous mushrooms, ?rose up slowly and majestically out of the ground and then split into pieces with a mighty roar, sending up multi-colored columns of flame mixed with a mass of earth and splinters high in the sky.” German losses that day included more than 10,000 men who died instantly, along with some 7,000 prisoners–men who were too stunned and disoriented by the explosions to resist the infantry assault.
Although Messines Ridge itself was a relatively limited victory, it had a considerable effect. The Germans were forced to retreat to the east, a sacrifice that marked the beginning of their gradual but continuous loss of territory on the Western Front. It also secured the right flank of the British thrust towards the much-contested Ypres region, the eventual objective of the planned offensive. Over the next month and a half, British forces continued to push the Germans back toward the high ridge at Passchendaele, which on July 31 saw the launch of the British offensive–known as the Battle of Passchendaele or the Third Battle of Ypres–in earnest.