Bulgaria is really an interesting country that isn't talked about much. I have visited it several times because I know some Bulgarians.
From what I have studied, I know it was a Roman province, then obviously with the division, it became part of the Eastern Roman Empire and underwent heavy domination by the Ottomans. However, it's not much mentioned in history books. It's overshadowed by Vlad in Romania, Greece, and Turkey, which have a more significant impact and surround it.
Bulgaria isn't prominent in world history books only because we (or rather, our politicians and "intelligentzia") go at great lengths to self-sabotage our cultural heritage.
If it wasn't for Khan Tervel in 8th century, Eastern Europe would've followed the same fate as Spain, falling entirely under Arabian influence, perhaps for centuries. During the High Middle Ages, Tarnovo, Bulgaria's then-capital, was considered "the third Rome" (the second one being Constantinople) in terms of cultural richness, sophistication, and amount of accumulated books in royal and clerical libraries. Before that, during Christianization, both the Catholic Church and the East-Orthodox Church fought over who would influence Bulgaria, and we played both sides in order to get the "best deal".
Then, there is the fact that some of our philo-theological schools have indirectly helped start the Renaissance itself in Europe, namely the katars, who had close ties with the bogomils after their exodus from Bulgaria. And the katars and Knights Templar are both the forefathers of the Renaissance.
The entire Slavic world owes the Cyrillic alphabet to Tzar Boris the First, one of the greatest Bulgarian kings in history, who commissioned it from St. Kiril after he left Byzantium.
We fought and humiliated the Latin Empire (made from crusaders that got a little too ambitious for their own good). We have kept the Ottomans at bay for
decades before they conquered the Balkans, and then have failed only because of internal strife and succession crisis in the midst of one of the largest invasions in history.
Also, we, as a people, have had
several countries all across Europe, all the way to far-east Asia, and even today in some of those regions (mostly modern Russia) you can find people who ethnically identify themselves as Bulgarians.
And so on and so forth.
Bottom line, Bulgaria is a country that may seem small and insignificant now, but we have played a key role multiple times in world history (and in the history of Europe especially). Lastly, Bulgaria is the only country in the world with the longest
unchanged name since its founding. Even China or India started out with different names.
