The Kingdom of Bulgaria participated in World War I on the side of the Central Powers from 14 October 1915, when the country declared war on Serbia, until 30 September 1918, when the Armistice of Thessalonica came into effect.
After the Balkan wars of 1912 and 1913, Bulgaria was diplomatically isolated, surrounded by hostile neighbors and deprived of Great Power support. Negative sentiment grew particularly in France and Russia, whose officials blamed Bulgaria for the dissolution of the Balkan League, an alliance of Balkan states directed against the Ottoman Empire. Bulgarian defeat in the Second Balkan War in 1913 turned revanchism into a foreign policy focus.
When the First World War started in July 1914, Bulgaria, still recovering from the economic and demographic damage of recent wars, declared neutrality. Strategic location and a strong military establishment made the country a desired ally for both warring coalitions, but its regional territorial aspirations were difficult to satisfy because they included claims against four Balkan countries. As the war progressed, the Central Powers of Austria-Hungary and the German Empire were in a better position to meet these demands. Bulgaria entered the war on the side of the Central Powers, invading Serbia in September 1915.
As the smallest of the Central Powers, Bulgaria made vital contributions to their common war effort. Its entry heralded the defeat of Serbia, thwarted the goals of Romania, and catalyzed the Ottoman war effort by providing a land and rail link from Germany to Istanbul.
Though the Balkan theatre saw successful campaigns of rapid movement by the Central Powers in 1915 and 1916, the conflict degraded into attritional trench warfare on both the Northern and the Southern Bulgarian Fronts after most Bulgarian goals were satisfied. This period of the war further damaged the economy, creating supply problems and reducing the health and morale of Bulgarian troops. Despite achieving national territorial aspirations, Bulgaria was unable to exit what otherwise would have been a successful war, weakening its will to continue to fight. These stresses intensified with time, and in September 1918, the multinational Allied armies based in Greece broke through on the Macedonian Front during the Vardar Offensive. Part of the Bulgarian Army quickly collapsed, and open mutiny followed as rebellious troops proclaimed a republic at Radomir. Forced to seek peace, Bulgaria requested an armistice with the Allies on 24 September 1918, accepting it five days later. For the second time in only five years, Bulgaria faced national catastrophe. Tsar Ferdinand I assumed responsibility, abdicating in favor of his son Boris III on 3 October.
The 1919 Treaty of Neuilly formally concluded Bulgaria's participation in World War I. Stipulations included the return of all occupied territories, the cession of additional territories and the payment of heavy war reparations.