Great Britian Declares War on Bulgaria

In the years following the achievement of complete independence Bulgaria became increasingly militarised: Dillon in 1920 called Bulgaria "the Prussia of the Balkans" In 1912 and 1913, Bulgaria became involved in the Balkan Wars, first entering into conflict alongside Greece, Serbia and Montenegro against the Ottoman Empire. The First Balkan War (1912–1913) proved a success for the Bulgarian army, but a conflict over the division of Macedonia arose between the victorious allies. The Second Balkan War (1913) pitted Bulgaria against Greece and Serbia, joined by Romania and Turkey. After its defeat in the Second Balkan War Bulgaria lost considerable territory conquered in the first war, as well as Southern Dobrudzha and parts of the region of Macedonia.

In the aftermath of the Balkan Wars, Bulgarian opinion turned against Russia and the western powers, whom the Bulgarians felt had done nothing to help them. The government of Vasil Radoslavov aligned Bulgaria with Germany and Austria-Hungary, even though this meant also becoming an ally of the Ottomans, Bulgaria's traditional enemy. But Bulgaria now had no claims against the Ottomans, whereas Serbia, Greece and Romania (allies of Britain and France) were all in possession of lands perceived in Bulgaria as Bulgarian. Bulgaria, recuperating from the Balkan Wars, sat out the first year of World War I, but when Germany promised to restore the boundaries of the Treaty of San Stefano, Bulgaria, which had the largest army in the Balkans, declared war on Serbia in October 1915. Britain, France and Italy then declared war on Bulgaria.

Against Serbia were marshalled the Bulgarian Army and two Austro-Hungarian armies, all under the command of Field Marshal Mackensen, totalling more than 682,000 soldiers. The Austro-Hungarians began their attack on October 7 with a massive artillery barrage, followed by attacks across the rivers. Then, on the 11th, the Bulgarian Army attacked from two directions, with their First Army from the north of Bulgaria towards Niš, the other with their Second Army from the south towards Skopje (see map). The First Army defeated the Serbian Second Army at the Battle of Morava, while the Bulgarian Second broke through the less trained and equipped Troops of New Areas and cut the vital railroad to Salonika (Battle of Ovche Pole). With the Bulgarian breakthrough, the Serbian position was hopeless; the main army in the north could try to retreat, or be surrounded and forced to surrender. In the Battle of Kosovo the Serbs made a last and desperate attempt to join the two incomplete Allied divisions that made a limited advance from the south, but were unable to gather enough forces, due to the pressure from north and east and were stopped by the Bulgarians under General Kliment Boyadzhiev and had to pull back.