South Slavic dialects historically formed a continuum. The turbulent history of the area, particularly due to expansion of the Ottoman Empire, resulted in a patchwork of dialectal and religious differences. Due to population migrations, Shtokavian became the most widespread in the western Balkans, intruding westwards into the area previously occupied by Chakavian and Kajkavian (which further blend into Slovenian in the northwest). Bosniaks, Croats and Serbs differ in religion and were historically often part of different cultural circles, although a large part of the nations have lived side by side under foreign overlords. During that period, the language was referred to under a variety of names, such as "Slavic", "Illyrian", or according to region, "Bosnian", "Serbian" and "Croatian", the latter often in combination with "Slavonian" or "Dalmatian".
What was the main cause of the spread of Shtokavian?
population migrations