It’s a messy sort of thing, working this out, especially in post-imperial contexts like the Balkans…
In a fascinating survey of travel writing and proto-nationalist writing in 19th-century Bulgaria Stoyan Raichevsky buils a case that the Pomaks, the name given to the roughly 300,000 Muslims living in Bulgaria, descendants of converts to Islam in the 17th and 18th centuries, were truly Bulgarian and knew it. They often lived side-by-side with Christian Bulgarians, repudiated the learning of Turkish, sang anti-Turkish songs, desperately tried to avoid serving in the Turkish army (p.75), sometimes supported resistance fighters, and, perhaps less importantly for the question of nationality, practised a local, syncretistic form of Islam.
‘The Muslims of the plains do not belong to the Turkish people. They are Christians who adopted Islam during the period of the oppression. It is true that they go to the mosque but in mixed areas they share with the Christians the same superstitions, almost the same azimas, and use the same amulets. Most of them speak Bulgarian and would return to their original religion with the same ease with which they deserted it. They don’t understand much of both the Koran and the Holy Scriptures’ (Albert Dumont in 1873, quoted on p.54).
Many ethnographers of the time called them Bulgarians, and it was the question of language that was most weighty in their mind: ‘Nationality is a community of people who are related by language, by origin, by tradition, by some common moral qualities, common sympathy, common aspirations… no matter whether they have political independence or not.’ (Petko Slaveikov writing in Mecedonia newspaper in 1867, quoted by Raichevsky on p.50)
But why were the Bulgarian Muslims treated so badly by the newly-independent Bulgarian state before the communist takeover? (It is perhaps easier to explain the brutalities of the communists – after all, they were communists!) Perhaps because these Bulgarian Muslims did not convert to Orthodoxy in great numbers after independence as many commentators had predicted.
For example, Austrian ethnographer and historian Felix Kanitz: ‘All religious hatred is alien to the Mohammedan Bulgarians, everywhere these Muslim Pomaks live in perfect harmony with their brothers by blood, the Christians. In the regions where they live together I never heard any complaint of animosity on both parts… [When the more numerous Christians establish their own rule over these regions] these crypto-Muslim Bulgarians will again return to the religion of their ancestors, the religion which they still practise secretly.’ (comment from 1877, quoted on pp.57-8)
Of course, the Russo-Turkish War of 1877-78 had a massive negative impact on the population and sparked off mass migrations of the Pomaks to Turkey. This was compounded by Christians’ greed for their property, Turkish imams propaganda at religious schools, fear of anti-Muslim recrimination (even though they were personally innocent of siding with the Turks – even the pro-Turkish British military leader St Clair couldn’t get Muslim Bulgarians to oppose the Russian occupation, pp.78-79) and bad administration by the new rulers (pp.65-72). Furthermore, when some Pomaks wanted to return, having not been treated well in Turkey and without the language, the new Bulgarian administration was not especially helpful (1909 order of the ministry of Foreign Affairs and Religion offered no special help to Bulgarian Muslims wanting to return home, p.86, a real missed opportunity).
Amusingly, Polish political intrigue of the time sought to convince the Pomaks that they were of Polish origin, and, presumably, should therefore be loyal to Poland rather than to their present Ottoman masters or to the nascent Bulgarian state or the Russian Imperial patron (p.52).