Bulgaria Declares War on Serbia
Bulgaria threw off the mask of neutrality on October 11, 1915, declaring war against Serbia on the pretext that the Serbians had crossed the frontier and attacked Bulgarian troops.
An army of 200,000 under General Boyadjieff, occupied a line along the frontier from Vidin to Zaribrod, threatening the Belgrade-Sofia Railroad. A second army, 100,000 strong, under General Teodoroff, faced toward Macedonia, with the railroad center of Uskub as its chief objective. By seizing Uskub, they would be able to drive a wedge into Serbia from east to west and close the natural Serbian path of retreat. Opposing these great armies on the Eastern frontier, Serbia had a force of 100,000 men.
Meanwhile, the vanguard of General Sarrail's army, 70,000 strong, had landed at Salonika, and though the treacherous King Constantine of Greece sought to intern them, the troops were at last permitted to advance through Greece to the relief of the Serbians. The Greeks hampered them, however, by rushing their own mobilization and seizing most of the cars on the main railroad needed for their transportation.
General Sarrail's Allied Army advanced up the Sardar expecting to form a junction with the Serbians. At Valandova, on October 15, 1915, an army of Bulgarians attacked them in force, but were repulsed with heavy losses. General Sarrail continued his advance to Kriwlak and Gradsko, but durst not go further until reinforcements arrived.
Desperate Fighting in the North
General Boyadjieffs Bulgarian Army in the North, meanwhile, had crossed the lower Timok River and advanced in two divisions toward the cities of Pirot and Kniashevats. Never was seen such savage fighting, such awful slaughter, as ensued during the next fortnight, when the Serbians and Bulgarians grappled with each other in those Serbian mountains. Whole regiments were hemmed in against the rocky walls and annihilated. In some of the combats, when guns were lost, the men seized boulders and hurled them at their opponents, or strangled their enemies with their grip. Quarter was neither asked nor given in this man-to-man combat. Numbers finally prevailed, however, and the Serbians were forced to yield village after village, ridge after ridge, the torch being applied to each town seized by the Bulgars.
Bulgars Cut the Railway and Seize Uskub
General Teodoroff's Bulgarian Army, moving rapidly into Macedonia, in two columns, aimed at seizing the main line of railroad and preventing communications between the vanguard of General Sarrail's French Army and the Serbians. One Bulgarian column cut the railroad at Vranya and occupied the city on October 17, 1915. Teodoroff's main army, going south, seized Palanka, Sultan Pepe and Katshaua, and advanced to Veles, where on October 20, 1915, they again cut the railroad line, making any further advance of General Sarrail's army impossible. Two days later the Bulgarians drove the Serbians out of Uskub and into the Katshanik Pass, where the Serbs made a gallant stand against great odds.
Mackensen's Offensive Begins
The entrance of Bulgaria into the War was the signal for General Mackensen to move his Austro-German Armies down from the North and encompass the Serbians. He found it a difficult task, despite his superiority in numbers and artillery. The Serbians, all along the line of the Danube and Save Rivers, had built parallel lines of strong intrenchments, protected by the rock-ribbed bilk skirting the mountains of the interior. The Germans, after crossing these rivers, had been held back by these obstacles. On October 15, 1915, Mackensen launched a powerful offensive all along the line. General Koevess moved out of Belgrade and bombarded the heights to the south. Mount Avala, ten miles away, succumbed in three days and by October 21, 1915, Obrenovatz and Shabatz were in his control.
Meanwhile, General Gallwitz was moving his army down the Morava Valley, using his heavy artillery against the fortified positions on the heights. By October 23, 1915, his army had advanced to the southern bank of the Jesenitza and had passed Rakinatz. On the extreme Western front the Austrians had crossed the Drina at Vishegrad and the German left wing had crossed the Danube at Ovsova, taking the heights overlooking the city. All the rivers forming the frontiers of Serbia were now in the hands of the German Allies, but they paid a heavy price for the gains.
The Kingdom of Bulgaria participated in World War I on the side of the Central Powers between 15 October 1915, when the country declared war on Serbia, until 29 September 1918, when the Armistice of Thessalonica was signed.