Because Illyrian-Celt tribes ( Dalmatie , Japods etc ) lived on present day Croatia , then the Slavic Croats arrived and assimilated those Illyrian-Celts , and on end we have Germanic influx during Frankish empire , Holy Roman Empire , Austro-Hungarian Empire and ww1 + ww2 period .
because illyrian celt tribes dalmatie japods etc lived on present day croatia then the slavic croats arrived and assimilated those illyrian celts and on end we have germanic influx during frankish empire holy roman empire austro hungarian empire and ww1 ww2 period