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.