Despite the establishment of the Principality of Bulgaria (subject to Ottoman suzerainty) in 1878, and the subsequent Bulgarian control over Eastern Rumelia after 1885, there was still a substantial Bulgarian population in the Balkans living under Ottoman rule, particularly in Macedonia.
To complicate matters, Serbia and Greece too made claims over parts of Macedonia, while Serbia, as a Slavic nation, also considered Macedonian Slavs as belonging to the Serbian nation.
Thus began a three-sided struggle for control of these areas which lasted until World War I. In 1903, there was a Bulgarian insurrection in Ottoman Macedonia and war seemed likely.

In 1908, Ferdinand used the struggles among the Great Powers to declare Bulgaria an independent kingdom with himself as Tsar.
He did this on 5 October (though celebrated on 22 September, as Bulgaria remained officially on the Julian Calendar until 1916) in the St Forty Martyrs Church in Veliko Tarnovo.
Even before then, however, Bulgaria had only acknowledged the overlordship of the sultan in a formal way.
Since 1878, Bulgaria had had its own constitution, flag and anthem, and conducted a separate foreign policy.

Ferdinand took the Bulgarian title "Tsar" in honor of the rulers of the First and Second Bulgarian Empires.
However, while previous Bulgarian "tsars" were reckoned as emperors, Ferdinand and his successors were called "king" outside Bulgaria.
 The Tarnovo Constitution was retained, with the word "tsar" replacing the word "prince."
Despite the establishment of the Principality of Bulgaria (subject to Ottoman suzerainty) in 1878, and the subsequent Bulgarian control over Eastern Rumelia after 1885, there was still a substantial Bulgarian population in the Balkans living under Ottoman rule, particularly in Macedonia. To complicate matters, Serbia and Greece too made claims over parts of Macedonia, while Serbia, as a Slavic nation, also considered Macedonian Slavs as belonging to the Serbian nation. Thus began a three-sided struggle for control of these areas which lasted until World War I. In 1903, there was a Bulgarian insurrection in Ottoman Macedonia and war seemed likely. 
In 1908, Ferdinand used the struggles among the Great Powers to declare Bulgaria an independent kingdom with himself as Tsar. He did this on 5 October (though celebrated on 22 September, as Bulgaria remained officially on the Julian Calendar until 1916) in the St Forty Martyrs Church in Veliko Tarnovo. Even before then, however, Bulgaria had only acknowledged the overlordship of the sultan in a formal way. Since 1878, Bulgaria had had its own constitution, flag and anthem, and conducted a separate foreign policy.
Ferdinand took the Bulgarian title "Tsar" in honor of the rulers of the First and Second Bulgarian Empires. However, while previous Bulgarian "tsars" were reckoned as emperors, Ferdinand and his successors were called "king" outside Bulgaria.  The Tarnovo Constitution was retained, with the word "tsar" replacing the word "prince."
Kingdom of Bulgaria