In July 1917, Field Marshal Sir Douglas Haig had begun the Third Battle of Ypres, an attempt to inflict unsustainable losses on the German army and to advance out of the Ypres Salient to capture the Belgian coast and end the submarine threat from Belgian bases to the southern North Sea and the Dover Straits. At the Battle of Messines Ridge (7–14 June) the ridge had been captured down to the "Oosttaverne" line and a substantial success had been gained at the Battle of Pilckem Ridge 31 July – 2 August. Ground conditions in August were poor, as the surface had been bombarded, fought over and partially flooded, at times severely so. Shelling had destroyed drainage canals in the area and unseasonable heavy rain in August turned some parts into morasses of mud and waterlogged shell-craters. Supply troops walked to the front on duck boards laid across the mud; carrying loads of up to , soldiers could slip into the craters and drown. Trees were reduced to blasted trunks, the branches and leaves torn away and corpses from earlier fighting were uncovered by the rain and shelling. The ground was powdery to a depth of and when wet, had the consistency of porridge.