[Sidenote: Main effort against Rancourt and Frégicourt.]
The general plan of the combined allied attack which was opened on September 15 was to pivot on the high ground south of the Ancre and north of the Albert-Bapaume road, while the Fourth Army devoted its whole effort to the rearmost of the enemy's original systems of defense between Morval and Le Sars. Should our success in this direction warrant it I made arrangements to enable me to extend the left of the attack to embrace the villages of Martinpuich and Courcelette. As soon as our advance on this front had reached the Morval line, the time would have arrived to bring forward my left across the Thiepval Ridge. Meanwhile on my right our allies arranged to continue the line of advance in close co-operation with me from the Somme to the slopes above Combles, but directing their main effort northward against the villages of Rancourt and Frégicourt, so as to complete the isolation of Combles and open the way for their attack upon Sailly-Saillisel.
A methodical bombardment was commenced at 6 a.m. on September 12, 1916, and was continued steadily and uninterruptedly till the moment of attack.
[Sidenote: Bombardment and infantry assault.]
At 6.20 a.m. on September 15, 1916 the infantry assault commenced, and at the same moment the bombardment became intense. Our new heavily armored cars, known as "tanks," now brought into action for the first time, successfully co-operated with the infantry, and, coming as a surprise to the enemy rank and file, gave valuable help in breaking down their resistance.
[Sidenote: Capture of the Quadrilateral.]
The advance met with immediate success on almost the whole of the front attacked. At 8.40 a.m. "tanks" were seen to be entering Flers, followed by large numbers of troops. Fighting continued in Flers for some time, but by 10 a.m. our troops had reached the north side of the village, and by midday had occupied the enemy's trenches for some distance beyond. On our right our line was advanced to within assaulting distance of the strong line of defense running before Morval, Les Boeufs, and Gueudecourt, and on our left High Wood was at last carried after many hours of very severe fighting, reflecting great credit on the attacking battalions. Our success made it possible to carry out during the afternoon that part of the plan which provided for the capture of Martinpuich and Courcelette, and by the end of the day both these villages were in our hands. On September 18, 1916, the work of this day was completed by the capture of the Quadrilateral, an enemy stronghold which had hitherto blocked the progress of our right toward Morval. Further progress was also made between Flers and Martinpuich.