Ferdinand I (Ferdinand Maximilian Karl Leopold Maria; 26 February 1861 – 10 September 1948) was Prince of Bulgaria from 1887 to 1908 and Tsar of Bulgaria from 1908 until his abdication in 1918. Under his rule, Bulgaria entered the First World War on the side of the Central Powers in 1915.
The Bosnian Crisis, also known as the Annexation Crisis (German: Bosnische Annexionskrise, Turkish: Bosna Krizi; Serbo-Croatian: Aneksiona kriza, Анексиона криза) or the First Balkan Crisis, erupted on 5 October 1908 when Austria-Hungary announced the annexation of Bosnia and Herzegovina, territories formerly within the sovereignty of the Ottoman Empire but under Austro-Hungarian administration since 1878.
This unilateral action – timed to coincide with Bulgaria's declaration of independence from the Ottoman Empire on 5 October – sparked protestations from all the Great Powers and Austria-Hungary's Balkan neighbors, Serbia and Montenegro. In April 1909, the Treaty of Berlin was amended to reflect the fait accompli and bring the crisis to an end. Although the crisis ended with what appeared to be a total Austro-Hungarian diplomatic victory, it permanently damaged relations between Austria-Hungary and its neighbors, especially Serbia, Italy and Russia, and in the long term helped lay the grounds for World War I. Austro-Serbian relations – harmed by the annexation's inflaming of Serbian nationalists – continued to be strained to the point of declaring war on each other in 1914.
The de jure independence of Bulgaria (Bulgarian: Независимост на България, romanized: Nezavisimost na Bǎlgariya) from the Ottoman Empire was proclaimed on 5 October [O.S. 22 September] 1908 in the old capital of Tarnovo by Prince Ferdinand of Bulgaria, who afterwards took the title "Tsar".