As of 1878, there were only three free Slavic states in the world: the Russian Empire, Serbia and Montenegro. Bulgaria was also free but was de jure vassal to the Ottoman Empire until official independence was declared in 1908. In the entire Austro-Hungarian Empire of approximately 50 million people, about 23 million were Slavs. The Slavic peoples who were, for the most part, denied a voice in the affairs of the Austro-Hungarian Empire, were calling for national self-determination. During World War I, representatives of the Czechs, Slovaks, Poles, Serbs, Croats, and Slovenes set up organizations in the Allied countries to gain sympathy and recognition. In 1918, after World War I ended, the Slavs established such independent states as Czechoslovakia, the Second Polish Republic, and the State of Slovenes, Croats and Serbs.
When were the Russian Empire, Serbia, and Montenegro the only free Slavic states in the world? (If the question is unanswerable, say "unanswerable")
1878