On 15 September Crown Prince Rupprecht, commander of the northern group of armies, was ordered to prepare a rear defensive line and on 23 September work on the new (Siegfried Position/Hindenburg Line) began. On 21 September, after the battle of Flers–Courcelette (15–22 September), Hindenburg ordered that the Somme front would have priority in the west for troops and supplies. By the end of the Battle of Morval (25–28 September) Rupprecht had no reserves left on the Somme. During September, the Germans sent another thirteen fresh divisions to the British sector and scraped up troops wherever they could be found. The German artillery fired of field artillery shells and of heavy ammunition, yet the début of the tank, the defeat at the Battle of Thiepval (26–28 September) and the number of casualties (September was the costliest month of the battle for the German armies) had been severe blows to German morale. On 7 October, Rupprecht anticipated a British attack north of the Ancre River in mid-October, anxiety about the situation at Verdun also increased and on 19 October, the dispatch of reinforcements from Verdun to the Somme was suspended. Defeats inflicted by the French Tenth Army (10–21 October) led to the sacking of Bronsart von Schellendorf, the 2nd Army chief of staff.