Faced with Russia, Austria-Hungary could spare only one-third of its army to attack Serbia. After suffering heavy losses, the Austrians briefly occupied the Serbian capital, Belgrade. A Serbian counter attack in the battle of Kolubara, however, succeeded in driving them from the country by the end of 1914. For the first ten months of 1915, Austria-Hungary used most of its military reserves to fight Italy. German and Austro-Hungarian diplomats, however, scored a coup by persuading Bulgaria to join in attacking Serbia. The Austro-Hungarian provinces of Slovenia, Croatia and Bosnia provided troops for Austria-Hungary, invading Serbia as well as fighting Russia and Italy. Montenegro allied itself with Serbia.

Serbia was conquered in a little more than a month. The attack began in October, when the Central Powers launched an offensive from the north; four days later the Bulgarians joined the attack from the east. The Serbian army, fighting on two fronts and facing certain defeat, retreated into Albania, halting only once to make a stand against the Bulgarians. The Serbs suffered defeat near modern day Gnjilane in the Battle of Kosovo. Montenegro covered the Serbian retreat toward the Adriatic coast in the Battle of Mojkovac in 6–7 January 1916, but ultimately the Austrians conquered Montenegro, too. Serbian forces were evacuated by ship to Greece.

In late 1915, a Franco-British force landed at Salonica in Greece, to offer assistance and to pressure the government to declare war against the Central Powers. Unfortunately for the Allies, the pro-German King Constantine I dismissed the pro-Allied government of Eleftherios Venizelos, before the Allied expeditionary force could arrive.

After conquest, Serbia was divided between Austro-Hungary and Bulgaria. Bulgarians commenced bulgarization of the Serbian population in their occupation zone, banishing Serbian Cyrillic and the Serbian Orthodox Church. After forced conscription of the Serbian population into the Bulgarian army in 1917, the Toplica Uprising began. Serbian rebels liberated for a short time the area between the Kopaonik mountains and the South Morava river. The uprising was crushed by joint efforts of Bulgarian and Austrian forces at the end of March 1917.

The Macedonian Front proved static for the most part. Serbian forces retook part of Macedonia by recapturing Bitola on 19 November 1916. Only at the end of the conflict were the Entente powers able to break through, after most of the German and Austro-Hungarian troops had withdrawn. The Bulgarians suffered their only defeat of the war at the Battle of Dobro Pole but days later, they decisively defeated British and Greek forces at the Battle of Doiran, avoiding occupation. Bulgaria signed an armistice on 29 September 1918.