A glance at the map will show that the main line of railroad, running down from Belgrade to Saloniki by way of Nish, passes within a few miles of the Bulgarian frontier, just opposite Sofia. Indeed, from Klisura on the frontier the distant whistle of the locomotives and the rattle of the trains across stretches of trestle work can be heard plainly on still days. From Klisura on the frontier to the railroad is all down hill. Farther south, at Kustendil, the danger was even greater, though the distance from frontier to railroad somewhat more, for at Kustendil was the terminus of a short railroad from the Bulgarian capital. From this point on the frontier toward the railroad at Kumanova the terrain was all in favor of the Bulgarians, for Kustendil is at the top of a chain of mountains and the railroad runs along the bottom of a valley, the famous Morava Valley. This railroad, from Upper Serbia down to Saloniki, was the only line of communication and transportation between the main Serbian armies and the Allies. Cut this, and they would wither like a flower separated from its stem. So keenly did the Serbians realize their danger that they asked permission of the Allies to attack Bulgaria before the Bulgarian army was completely mobilized. They hoped thereby to disable Bulgaria with one sharp blow while she was not yet prepared, then turn their whole attention toward the enemy in the north. But to this plan the Allies would not consent, still hoping that Ferdinand would reconsider his resolution. [Illustration: General Map of Balkan (Serbian) Operations.] Just before the fourth invasion actually began, the Serbians held their frontier along the Danube and the Save with three armies, consisting of nearly eight divisions, or half of all their available men. On the west the First Serbian Army, of three divisions, commanded by General Mishitch, occupied the angle formed by the Save and the Drina, with its headquarters at Shabatz, the scene of such bloody fighting a year before. To the eastward came a force of a division and a half under command of General Zivkovitch, known as the Army for the Defense of Belgrade, which indicates its position. Between Belgrade and the Rumanian frontier lay the Third Serbian Army, of three divisions, with General Jourishitch at its head, protecting the mouth of the Morava Valley. Facing the Austrians over in the west, in the vicinity of Vichegrad, was the army of Ushitze, of less than two divisions, under General Goykovitch. These were the forces, about two-thirds of the total Serbian army, which faced the Austro-Germans. But another 100,000 had also to be deployed along the Bulgarian frontier to protect the railroad as best they could. Thus it was that wherever she faced her enemies, Serbia, was hopelessly outnumbered. CHAPTER XXXI THE INVASION BEGINS