 On 6 October 1915, less than a year after Serbia marked the first Allied victory of World War One and humiliated its powerful neighbour, the Austro-Hungarian Third Army, the German Eleventh Army, as well as General Sarkotić’s army from Bosnia, began the fourth invasion of Serbia.The overwhelming superiority in heavy artillery, as well as the weight of numbers, quickly overwhelmed the Serbian army who started streaming southwards towards Kragujevac and Niš; Five days later, the Serbs were caught by surprise when the Bulgarian First and Second Armies invaded Serbia from the east, cutting the rail line that ran north from Salonika and depriving Serbia of reinforcements and artillery ammunition.The Serbian high command was forced to transfer its all-important Second Army from the northern front to defend this border.