The Serbian army, coming up from the south of the country, met the Austrian army at the Battle of Cer on 12 August.
The Serbians occupied defensive positions against the Austrians. The first attack came on August 16, between parts of the 21st Austro--Hungarian division and parts of the Serbian Combined division. In harsh night-time fighting, the battle ebbed and flowed, until Stepa Stepanovic rallied the Serbian line. Three days later the Austrians retreated across the Danube, having suffered 21,000 casualties as against 16,000 Serbian. This marked the first major Allied victory of the war. The Austrians had not achieved their main goal of eliminating Serbia, and it became increasingly likely that Germany would have to maintain forces on two fronts...
The Battle of Cer (August 12-24. 1914) was one of the most glorious in the military history of Serbian people. Victory achieved by the Serbian Army in this battle was the first allied victory in the First World War (1914-1918). In the crucial part of the operation, on the Mt. Cer, Austro-Hungarian Fifth Army was defeated and pushed back across River Drina.
The Battle of Drina (September 6. - November 11. 1914) was the most difficult battle waged by the Serbian Army in WWI, particularly battles around middle flow of River Drina, battles of Gucevo and Mackov Kamen. It also stopped the second offensive of Austro-Hungarian Army on Serbia.
The Battle of Kolubara (November 16. - December 15. 1914) and the victory of the Serbian Army contributed to Serbian respect among the allies. In the final phase of the battle, in only 13 days, the Serbian Army managed to expel the enemy from the country and re-establish the fronts on Drina and Sava rivers.
The Battle near Mojkovac (January 6-7. 1916), in which Montenegrin Sandzak Army successfully defended itself against Austro-Hungarin offensive, significantly alleviated the operations of the Serbian Army, enabling it to withdraw its troops trough Montenegro towards Albania.
Albanian Golgotha - Retreat of the Serbian Army (November 1915 - January 1916), also known as "Serbian Golgotha through Albanian gorges" was carried out in the conditions of strong frost, hunger, fatigue, illness and almost every day battles against Bulgarians, Austria-Hungary and Albanians. Some 100,000 soldiers and refugees lost their lives during this legendary march-maneuver of the Serbian Army, which was compared by various historians with Napoleon's and Suvorov's crossing the Alps.
Thessaloniki front (1916-1918) encompasses battles waged by the Entente forces against Central forces on the territory from the Orfan Bay, across Greece and Albania, to the Ionian Sea. In September 1916, the Serbian Army took part in the allied offensive, occupying Kajmakcalan after fierce struggle against Bulgarian forces.
Breakthrough of the Thessaloniki front and allied offensive in autumn 1918, in which the Serbian Army played a crucial role, belong to the most successful operations of the WWI.
Its participation in the WW I Serbia paid with more than million people (some 22% of the population, 58% of male population) and Montenegro lost around 50,000 people (1/8of the entire population). From 707.000 mobilized men during the war, only less than 130.000 returned home at the end of the war.