Yes they DO (Croatians) have Slavic origins, as their language would also suggest and genetics also upholds (20-35%) R1a. But these more recent Slavic invaders, coming from the Russian steppes and the Ukraine are similar to the Slavs that pushed west into Europe reaching such places as Poland, Slovakia, parts of Czech Republic and affecting Slovenians etc. in my opinion they sort of "mixed in" with the local indigenous haplogroup I2a1b men of the Balkans. This is what it is to be Croatian, some of them hold the genetic legacy of the indigenous haplogroup I men (40 or so percent) and others arrived more recently with the Slavic migrations and brought their immigrant language with them but, clearly, these R1a men where not the overwhelming majority, they significantly impacted Croatia but they are not as much as in Poland Ukraine, Belarus or Russia. Can someone correct me if my theory is invalid or incorrect? : )