In June, the British launched an offensive in Flanders, in part to take the pressure off the French armies on the Aisne, after the French part of the Nivelle Offensive failed to achieve the strategic victory that had been planned and French troops began to mutiny. The offensive began on 7 June, with a British attack on Messines Ridge, south of Ypres, to retake the ground lost in the First and Second battles in 1914. Since 1915 specialist Royal Engineer tunnelling companies had been digging tunnels under the ridge, and about of explosives had been planted in 21 mines under the German defences. Following several weeks of bombardment, the explosives in 19 of these mines were detonated, killing up to 7,000 German troops. The infantry advance that followed relied on three creeping barrages which the British infantry followed to capture the plateau and the east side of the ridge in one day. German counter-attacks were defeated and the southern flank of the Gheluvelt plateau was protected from German observation.