up with Austro-Hungarian forces advancing from Orșova and secure the Belgrade—Sofia railway by capturing Niš, war-time capital of Serbia. The Bulgarian Army crossed the border on October 11. But the first two weeks of the offensive the advance was slow as the divisions of the army operated independently towards achieving their individual objectives. In the southern sector the "Sofia" infantry division was engaged in heavy fighting with parts of the Serbian Second Army and failed to take Pirot. However, the Bulgarian advance in the north was proceeding more rapidly and the taking of Knjaževac by the "Tundzha" division eventually forced the Serbians to pull out of Pirot in order to avoid encirclement. This was a minor success for the Bulgarians which captured 14 cannons and c. 1,500 prisoners but the main Serbian forces were allowed to retreat in good order. Faced with the war on several fronts the Serbian High Command decided that it is best for the army to try to hold its opponents for as long as possible and retreat slowly to the south, southwest until help arrived from the Allies who had landed in Thessaloniki. Under these circumstances the renewed drive of the Bulgarian Army faced determined resistance but it was not enough to prevent the fall of Aleksinac and on 23 October (5 November) 1915 the 9th "Pleven" division entered Niš where the Serbians had left some 42 old fortress cannons, several thousand rifles and a big amount of ammunitions, in addition around 5,000 soldiers surrendered or were captured by the Bulgarians. The Belgrade–Sofia railway was now opened for the Central Powers and a permanent land connection was established. The Serbians, however, managed to destroy the bridges of the Morava river and slowed significantly the Bulgarian First Army which spend the time between 24 and 31 October in unsuccessful attempts to cross the river. This marked the end of the Morava Offensive. Having suffered heavy losses the Serbian Army retreated towards Kosovo where they prepared to make their last stand. The Central Powers meanwhile prepared a plan to encircle the Serbian forces. The Bulgarian High Command and the headquarters of Army Group "Mackensen" agreed to pursue relentlessly the retreating opponent, to cut its possible retreat routes through Macedonia and to undertake a decisive advance towards Pristina and prevent a Serbian retreat to the west. The plan required the Bulgarian First Army to attack from the east,