The Bulgarian Government suddenly threw aside all dissimulation and declared war on Serbia, on the pretext that the Serbians had crossed the frontier and attacked Bulgarian troops. On October 11, 1915, the Bulgarian army began operations by attacking the Serbians at Kadibogas, northwest of Nish, the attack gradually extending up and down the frontier. This was the fatal blow. To oppose the 300,000 men that the Bulgarians could easily put into this field, the Serbians had not over a third as many. Bulgaria had two large armies against the Serbian frontier. The First Army, under General Boyadjieff, was fully 200,000 strong and was concentrated in the north from Vidin to Zaribrod, threatening the Timok Valley and that part of the Belgrade-Sofia railroad running from Pirot to Nish. The Second Army, under the command of General Todoroff, was only half as large, and directed itself toward Macedonia and especially toward Uskub, both on account of the strategic importance of that place as a railroad center and as the best point from which a wedge might be driven into the side of Serbia, separating the north from the south. The headquarters of this second force was in Kustendil, its left wing extending down to Strumitza in Macedonia. On this eastern front, to oppose the Bulgarians, the Serbian forces were in three groups. In the north, its left flank touching the forces operating against the Austro-Germans, lay the Timok group, commanded by General Zivkovitch, whose headquarters were in Zaichar. South of this force came the second group--territorial troops--numbering three divisions of infantry and one of cavalry, altogether about 80,000 men, and commanded by Marshal Stepanovitch. It was based on Pirot and was especially charged with the defense of the railroad. Lower down, with headquarters in Vranya, was the detachment of the Southern Morava. Farther down in Macedonia, concentrated around Uskub, Veles, and stretched down along the Vardar toward the Greek frontier at Doiran, were another 25,000 men under the command of General Bojovitch. [Illustration: Under fire from the Serbian forces, General Mackensen's engineers constructed this great bridge across the Danube, and his army crossed for the invasion of Serbia.]