In December 1915, Augustin Dubail commanding (GAE: Eastern Army Group) planned a larger operation to consolidate the French position in the region by capturing Mulhouse. An attack on Hartmannswillerkopf by the 66e Division (General Marcel Serret), which had been fighting in the area all year was to be the prelude to the larger attack. The division was given another 250 guns for the attack, two of which were super-heavy 370 mm Filloux mortars, an average of one gun per of German front. After several postponements, the French bombardment began on 21 December from Hartmannswillerkopf to Wattwiller. In the afternoon the 66e Division attacked, taking the peak and trenches at Hirtzstein to the north-west of Wattwiller as German reserves established a new front line on the eastern slopes.