It was this Bulgarian stab in the back which made havoc of the Serbian defence. Mackensen made slow progress at first, partly because he had no wish to drive the Serbians south until the Bulgars had cut off their retreat down the Vardar. Belgrade did not fall until three weeks after the bombardment had opened; but with the intervention of the Bulgarian armies all along the bare Serbian flank, events moved with tragic rapidity. On 17 October the Salonika line was pierced at Vrania, Veles fell on the 20th, and Uskub on the 21st. By the 26th Mackensen and the Bulgarians had effected a junction in the north and cleared the Danube route into the Balkans. Nish fell on 5 November after three days' fierce fighting, and the Constantinople railway thus passed into enemy hands. In the north-west the Austrians were pressing on from Ushitza down by the Montenegrin frontier towards Mitrovitza, threatening to crush the Serbians on the Kossovo plateau between them and the Bulgars. To save the main Serbian force and keep open a retreat through Albania, a stand had to be made at Katchanik against the Bulgars advancing north from Uskub. It was successful to that extent, and when at one moment the Serbs temporarily broke the Bulgarian front, a junction seemed possible with the French advance from Veles. But both Allies were too weak for the solid Bulgarian wedge. The Serbs had to fall back from Kossovo and the French to their entrenched camp at Kavadar. A still narrower chance intervened between the French and the Serbs who were fighting at the Babuna Pass to bar the way to Prilep and Monastir, and at one time the French flung out their left to within ten miles of the Serbian position. But their own communications were threatened all down the narrow line of the Vardar, and they were hopelessly outnumbered by the Bulgarian forces. Retreat was the common misfortune and necessity. Prilep fell on 16 November; and farther north, as the Serbians retreated into Montenegro and Albania, the Austrians occupied Novi Bazar on 20 November, and Mitrovitza and Prishtina on the 23rd. On the 28th the Germans announced that "with the flight of the scanty remnants of the Serbian army into the Albanian mountains our main operations are closed."