No important operation was attempted by the British troops around Lens for several days following, but bombardments were incessant. The Canadians, though constantly under fire and subjected day and night to strong attacks, firmly held their positions. The situation of the Germans in the center of Lens with an active enemy occupying high ground to the north and south was not a comfortable one. When they did fight it was with rage and despair. Prisoners taken told of the deadly effects of the British gas projectiles which wrought awful havoc among the troops herded in tunnels and dugouts. The constant bombardment of Lens had reduced the city to the same ruined condition as the suburbs. The subterranean caverns which the Germans had built by the forced labor of civilians two years and more before became death traps. Often they collapsed under a heavy bombardment, burying alive whole companies in the ruins. Again, as frequently happened, a shell would explode inside the caves with fatal results to the occupants.
The Germans had every reason to wish that they had never introduced poison gas in warfare, for as employed by the British it had become a far deadlier destructive weapon than in their own hands. Many times during these days the Canadians had flooded the city with gas which soaked down into the tunnels and dugouts and stifled men in their sleep, or they died with their masks on if they delayed a second too long.
After days of raids and counterattacks insignificant when considered as separate operations, but important in the aggregate, British troops under Field Marshal Haig delivered a powerful attack against the German lines east of Ypres on an eight-mile front on September 20, 1917. According to an eyewitness, the barrage that preceded the advance was the heaviest on record.
The British were successful in attaining all their objectives. The German center along the Ypres-Menin road was penetrated to the depth of a mile.
Rain had fallen steadily during the night preceding the assault, but the assembling of the regiments assigned to the operation was carried through without a mishap. North Country troops carried the Inverness Copse. Australian troops stormed Glencorse Wood and Nonneboschen (Nun's Wood); Scottish and South African brigades captured Potsdam, Vampire, and Borey Farms; West Lancashire Territorials carried Iberian Farm and the strong point known as Gallipoli. After making a swift "clean-up" of these places and positions the British forces advanced on the final objectives.