Complex sentence: There was a systematic ethnic cleaning (killings, forced expulsions etc.) of the very large number of Muslims who had lived peacefully with neighbours in Hungary, Transylvania and Slovenia and very few if any Muslims were left in the areas conquered by the Austrian Imperials.
Split into simpler sentences: There was a systematic ethnic cleansing (killings, forced expulsions, etc.) of the very large number of Muslims who had lived peacefully with neighbours in Hungary, Transylvania and Slovenia. Very few if any Muslims were left in the areas conquered by the Austrian Imperials.