In the aftermath of the Balkan Wars, Bulgarian opinion turned against Russia and the Western powers, by whom the Bulgarians felt betrayed. The government of Vasil Radoslavov aligned Bulgaria with the German Empire and Austria-Hungary, even though this meant 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) held lands perceived in Bulgaria as Bulgarian.

Bulgaria sat out the first year of World War I recuperating from the Balkan Wars.^[43] Germany and Austria realized they needed Bulgaria's help in order to defeat Serbia militarily thereby opening supply lines from Germany to Turkey and bolstering the Eastern Front against Russia. Bulgaria insisted on major territorial gains, especially Macedonia, which Austria was reluctant to grant until Berlin insisted. Bulgaria also negotiated with the Allies, who offered somewhat less generous terms. The Tsar decided to go with Germany and Austria and signed an alliance with them in September 1915, along with a special Bulgarian-Turkish arrangement. It envisioned that Bulgaria would dominate the Balkans after the war.^[44]

Bulgaria, which had the land force in the Balkans, declared war on Serbia in October 1915. Britain, France and Italy responded by declaring war on Bulgaria. In alliance with Germany, Austria-Hungary and the Ottomans, Bulgaria won military victories against Serbia and Romania, occupying much of Macedonia (taking Skopje in October), advancing into Greek Macedonia, and taking Dobruja from Romania in September 1916. Thus Serbia was temporarily knocked out of the war, and Turkey was temporarily rescued from collapse.^[45] By 1917, Bulgaria fielded more than a quarter of its 4.5 million population in a 1,200,000-strong army,^[46] and inflicted heavy losses on Serbia (Kaymakchalan), Great Britain (Doiran), France (Monastir), the Russian Empire (Dobrich) and the Kingdom of Romania (Tutrakan).

However, the war soon became unpopular with most Bulgarians, who suffered great economic hardship and also disliked fighting their fellow Orthodox Christians in alliance with the Muslim Ottomans. The Russian Revolution of February 1917 had a great effect in Bulgaria, spreading anti-war and anti-monarchist sentiment among the troops and in the cities. In June Radoslavov's government resigned. Mutinies broke out in the army, Stamboliyski was released and a republic was proclaimed.