War and disease had reduced the Serbian armies before the campaign began to some 200,000 men, and their enemies brought at least double that number against them. The Serbians were, moreover, constrained by the counsels of the Allies to preserve what they could of their forces as a nucleus for future resistance, and thus to stand only so long as retreat remained open. Threatened on three sides by superior numbers, they were in an untenable position and had a well-nigh impossible task; and only skill, endurance, and courage brought the remnants out of the death-trap laid by collusion between the Central Empires and Bulgaria. The campaign was for the Serbians simply a series of rearguard actions encouraged at first by the delusive hope that the Allies might yet be in time. They might have been, had they been numerous enough. The French from Cape Helles came first, landed at Salonika on 5-7 October, and by the 27th had occupied the valley of the Vardar as far north as Krivolak. They also seized the commanding heights of Kara Hodjali north-east of the river, and repulsed the Bulgarian attempts to drive them off in the first week of November; while to the west they stretched out a hand towards the Serbians defending the Babuna Pass. With adequate forces they could have pushed beyond Veles to Uskub, broken the wedge which the Bulgarians had driven in between them and the Serbians, restored the line of the Vardar, and secured the Serbian retreat. 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