 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.In spite of a tenacious defense that caused the enemy to advance slower than anticipated, the pressure of the Austro-Hungarians, the Germans, and the Bulgarian First Army in the north and the Bulgarian Second Army advancing from the east forced the Serbs to retreat in a southwesterly direction into Kosovo.