At the start of the First World War Serbia had successfully defeated two attempted Austrian invasions. Over most of the next year the Austrians had been obsessed with more important campaigns against the Russians. Serbia’s safety relied on a continued Russian threat north of the Carpathians. That changed after the German victory at Gorlice-Tarnow (2-10 May 1915). The Russians were forced to evacuate Poland, and the threat to the Austro-Hungarian Empire receded.
The Central Power’s successes in Poland and the Allied failure at Gallipoli encouraged Bulgaria to join the Central Powers. She had territorial claims against Serbia, On 6 September Bulgaria, Germany and Austria signed an agreement in which they agreed to be ready to invade Serbia at the start of October – the Germans and Austrians with twelve divisions, the Bulgarians with four of their larger divisions. The eventual invasion force would contain six Bulgaria, seven Austrian and ten German divisions. In all twenty three divisions, with 330,000 men and 1,200 guns, would attack 11 Serbian divisions, with 200,000 men and 300 guns.
The earlier Austrian invasions had been made from the west. This time the Germans planned to attack from the north and east. Austrian Third Army (Kösvess) would attack west of Belgrade, the German Eleventh Army (Gallwitz) east of the city. The main thrust would be made on a 50 mile front centred on Belgrade. To the east the Bulgarian First Army would attack towards Nish, in the east of Serbia, with the Second Army further south to cut the railway to Salonika. If the Serbs concentrated against the main thrust in the north, then they would be cut off from their best line of retreat.
The Bulgarians announced their mobilisation on 23 September. The Serbs called for Allied assistance, but the only possible route the British and French could use was through Greece. The Greek Prime Minister Venizelos was pro-Allied and issued them with an invitation to land an army at Salonika. On 5 October he was sacked by King Constantine, who was the Kaiser’s brother in law, and favoured neutrality. Despite this, on the same day the Allies landed at Salonika.
On the same day the German and Austrian bombardment began across the rivers Sava and Danube, west and east of Belgrade. Two days later both armies began to cross the rivers, and on 9 October Belgrade fell. The Bulgarian attack began on 11 October and soon began to threaten the railway line to Salonika.
The Serbs formed a new line south of the Danube. After a brief delay the German and Austrian offensive was renewed on 18 October. The Serbs held out for the first day, but were then were overwhelmed and forced to retreat. On 23 October their right flank was hit by the Bulgarians advancing from the east. By that point the Salonika railway had already been cut, and a half-hearted Allied advance stopped in its tracks.
The Germans now hoped to envelope the retreating Serb armies in the middle of Serbia, but the Serbs refused to get drawn into hopeless defensive battles, and instead kept up a determined fighting retreat. Nish fell on 5 November, and by the middle of the month the Serbs had been pushed back to Kosovo. With the Bulgarians blocking their retreat south, the Serbs turned west.
Their route west cut across the mountains to the Adriatic. Some 140,000 Serb soldiers reached the Albanian border in December after a desperate winter trek through the mountains. Once on the Adriatic coast they were rescued by the Allied navies (mostly by the neighbouring Italians) and taken to Corfu where they were reequipped and reorganised, before eventually taking part in the ongoing Salonika campaign.
The victory over Serbia caused great satisfaction amongst the Central Powers. The Kaiser and King Ferdinand of Bulgaria held a great celebration at Nish on 18 January 1916, the 45th anniversary of the founding of the German Empire. At the start of the third year of the war the Russians had been thrown back and the Serbs eliminated. The Germans were free to turn their attention to the Western Front, where they would soon launch their first major offensive since 1914, at Verdun.