Bulgaria during World War I

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 belligerent parties signed the Armistice of Thessalonica.

In the aftermath of the Balkan wars of 1912 and 1913, Bulgaria found itself isolated on the international scene, surrounded by hostile neighbors and deprived of the support of the Great Powers. Anti-Bulgarian sentiment grew particularly in France and Russia, whose political circles blamed the country for the dissolution of the Balkan League, an alliance of Balkan states directed against the Ottoman Empire. The failure of Bulgarian foreign policy turned revanchism into a focus of Bulgaria's external relations.

When the First World War started in July 1914, Bulgaria, still recovering from the negative economic and demographic impact of recent wars, avoided direct involvement in the new conflict by declaring neutrality. Strategic geographic location and a strong military establishment made the country a desired ally for both warring coalitions, but Bulgaria's regional aspirations were difficult to satisfy because they included territorial claims against four Balkan countries. As the war progressed, the Central Powers of Austria-Hungary and the German Empire found themselves in a better position to fulfil Bulgarian demands and persuaded the country to join their cause in September 1915.

Though the smallest member of the Central Powers in area and in population, Bulgaria made vital contributions to their common war effort. Its entry to the war heralded the defeat of Serbia, thwarted the foreign-policy goals of Romania, and ensured the continuation of the Ottoman war effort by providing a geographical conduit for material assistance from Germany to Istanbul.

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Wikipedia