Prince Ferdinand I declared Bulgaria's complete independence from the Ottoman Empire, upgrading the country from an autonomous principality to a sovereign kingdom with himself as Tsar. The move exploited Ottoman weakness following the Young Turk Revolution and set off a diplomatic scramble in the Balkans, with Austria-Hungary simultaneously annexing Bosnia and Herzegovina days later.
For thirty years Bulgaria had existed in an awkward limbo, technically a vassal principality of the Ottoman Sultan even though it ran its own affairs. Ferdinand, a Saxe-Coburg prince who had ruled since 1887, decided the fiction had run its course. On September 22, 1908, in the old capital of Tarnovo, he stood before a crowd and read a manifesto declaring Bulgaria an independent kingdom, crowning himself Tsar in deliberate echo of the medieval Bulgarian empire.
The timing was shrewd and cynical. The Ottoman Empire had just been rocked by the Young Turk Revolution, leaving Constantinople in no position to object forcefully. Ferdinand had also quietly arranged financial compensation and secured Russian sympathy beforehand. Austria-Hungary used the same window of Ottoman distraction to formally annex Bosnia-Herzegovina, a move that infuriated Serbia and Russia and became one of the tripwires leading toward the First World War.
Bulgarians celebrated wildly, but the declaration also locked the young kingdom into the Balkan power struggles that would consume it in two Balkan Wars within four years, and then draw it into the catastrophe of 1914.