day, Supreme Army Command informed the leaders in Berlin that armistice talks were inevitable. The final assault on the Hindenburg Line began with the Meuse-Argonne offensive, launched by American and French troops on 26 September. The following week, co-operating American and French units broke through in Champagne at the Battle of Blanc Mont Ridge, forcing the Germans off the commanding heights, and closing towards the Belgian frontier. On 8October the line was pierced again by British and Dominion troops at the Battle of Cambrai. The German army had to shorten its front and use the Dutch frontier as an anchor to fight rear-guard actions as it fell back towards Germany. When Bulgaria signed a separate armistice on 29 September, Ludendorff, having been under great stress for months, suffered something similar to a breakdown. It was evident that Germany could no longer mount a successful defence. The collapse of the Balkans meant that Germany was about to lose its main supplies of oil and food. Its reserves had been used up, even as US troops kept arriving at the rate of 10,000 per day. Breakthrough of Macedonian Front Allied forces started the Vardar offensive on 15 September at two key points: Dobro Pole and near Dojran Lake. In the Battle of Dobro Pole, the Serbian and French armies had success after a three day long battle with relatively small casualties, and subsequently made a breakthrough in the front, something which was rarely seen in World War I. After the front was broken, Allied forces started to liberate Serbia and reached Skopje at 29 Septembar after which Bulgaria signed an armistice with the Allies on 30 September. German Emperor Wilhelm II wrote a telegram to Bulgarian Tsar Ferdinand I: "Disgraceful! 62,000 Serbs decided the war!". Allied armies continued the liberation of Serbia while Germany unsuccessfully tried to establish new front lines near Niš by sending troops from Romania. After Serbian army entered Niš on 11 October, Germany left Austro-Hungary to organize the Balkan front. On 1 November Serbian forces liberated Belgrade and started to cross over the border with Austria-Hungary. Austria-Hungary was politically disintegrating and signed an armistice with Italy on 3 November, leaving Germany alone in Europe. On 6 November the Serbian Army liberated Sarajevo and Novi Sad on 9 November. The non-German peoples of Austria-Hungary started to organize independent states in the territory of Austria-Hungary, which it was unable to