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.]

As a slight offset to the disheartening news that the Bulgarians had
at last definitely joined hands with the Teutonic forces, came the
tidings that France and England had declared war on Bulgaria and that
their forces, which had been landing in Saloniki, were already
advancing up the Vardar with the intention of making a junction with
the southern Serbian forces. Already, on that same