Fighting between Bulgaria and Serbia begins

The British chronology of the war records under 11 October 1915 the beginning of hostilities between Bulgarian and Serbian troops. War had not yet been declared, but the first shots were fired on the frontier. According to the Austro-Hungarian official history, on that day the Bulgarians provoked a clash with Serbian covering troops in the north-western corner of their country in order to have a reason to start the war. The government in Sofia then announced that Bulgaria had been provoked and was in a state of war with Serbia. The Bulgarian version was the reverse: it was the Serbs who had supposedly attacked Bulgarian territory.

Sources differ as to when the border skirmishes turned into a real attack. The English journalist Gordon Gordon-Smith, who was then at Serbian headquarters, wrote that on 12 October the Bulgarian armies attacked along the whole line without a declaration of war. An American calendar of the war from those weeks recorded a Bulgarian attack at three points on 13 October. According to Serbian sources the main attack began on the night of 13–14 October, and the Austro-Hungarian official history states that both Bulgarian armies went over to the attack on 14 October, the day of the formal declaration of war.

Armies on the frontier

Bulgaria had just completed the mobilisation begun at the end of September. Two armies stood on the Serbian frontier. The First Army of General Kliment Boyadzhiev, subordinated to Field Marshal August von Mackensen, was to strike through Knjaževac at Niš, take fortified Zaječar and Pirot and clear the bank of the Danube near Negotin to open the waterway to Turkey. The Second Army of General Georgi Todorov, directly subordinate to the Bulgarian command, was to enter the valley of the Vardar and cut the railway from Niš to Salonika, the only route by which Serbia could receive help from its allies.

Facing them stood, among others, the Serbian Timok Group and the Second Army of Vojvoda Stepa Stepanović, defending the mountain passes above Pirot. In Macedonia the Serbs had only weak forces. As late as the end of September Serbia had kept roughly half its army on the Bulgarian frontier, and its commander-in-chief, Vojvoda Radomir Putnik, had wanted to strike at the Bulgarians before they finished mobilising. The Allies did not agree, because they still hoped that Sofia could be dissuaded from war.

Serbia in a vice

Since 6 October Serbia had already been repelling the German and Austro-Hungarian offensive from the north. On 9 October Belgrade fell, and Mackensen’s armies were bringing more divisions across the Sava and the Danube. The Serbian command in Kragujevac had to choose which front to weaken. On 13 October it decided to transfer part of its forces from the east and west to the northern front. This weakening of the Bulgarian frontier was soon to take its revenge.

That same day Putnik learned from the French military attaché that General Maurice Sarrail, who had taken command at Salonika, did not intend to send the newly unloaded troops north in small detachments. He wanted to assemble his whole force first. The Serbs, meanwhile, were advised to do everything in their power to prevent Mackensen’s armies from joining up with the Bulgarians. On 14 October Bulgaria formally declared war on Serbia and its armies crossed the frontier.