On this day in 1915, the Kingdom of Bulgaria joins the Central Powers and enters the First Word War. The country official joins the war after an announcement by the Bulgarian Prime Minister. Sofia had been courted by both sides in the early stages of the war by both the German-dominated Central Powers and the Western Allies. The entry of Bulgaria into the war allowed the Central Powers to have a strategic advantage in the Balkan region. The Bulgarians, who had a large army for a country of its side, joined the war on behalf of the Central Powers because it was reliant on trade with Austro-Hungary and the Ottoman Empire. The main reason why Bulgaria joined the war with Germany, Austro-Hungary, and Turkey was to inflict a defeat on its great foe Serbia. The Serbs were trying to repel an Austro-Hungarian invasion and this allowed the Bulgarians to seize the disputed province of Macedonia. This former Ottoman province was the scene of a bitter conflict between Bulgaria and Serbia. Here both Belgrade and Sofia sponsored different militias and terrorist groups in a bid to dominate the province. Macedonia in the years before WW I was ravaged by terrorist attacks by rival groups sponsored by Belgrade and Sofia.
After the declaration of war, the Bulgarian army swiftly occupied Macedonia and it forces then attacked Serbia. The entry of Sofia into the war made the situation in Serbia very precarious and it was to contribute to its eventual defeat in 1916.
The Bulgarian government received sizable territories at the expense of Serbia but this was not enough it later seized some territory in neutral Greece and only an allied air and naval attack prevented a full-scale Bulgarian invasion of Greece. From 1916 the Bulgarians and the Greeks with their respective allies confronted each other along the Macedonian or Salonika Front in a bloody stalemate. Bulgaria was defeated in the summer of 1918. The French, Greek and exiled Serbia army units broke through the Bulgarian positions in Macedonia, after an intensive artillery bombardment. The Bulgarian troops were very demoralized at this stage and many deserted during the bombardment. The allies began to seize large swathes of territory, and this along with growing discontent in the country meant that Bulgaria was in no position to fight on. There was a near revolutionary situation in the country and many army units had mutinied. The King even feared a communist revolution. Sofia was forced to withdraw from the war and signed an armistice with the allies. Soon after there was a revolution in Sofia and a new more democratic government came to power. Some 100,000 Bulgarian soldiers died in the war.