>>213965972
Extraordinarily old animosity that has been going for almost 2000 years, Serbs and Croats both used to war each other in the dark ages, both used to blame each other for many WW2 warcrimes and for the Nazi\USSR doing whatever they wanted in the Balkans, both blame each other for the fall of Yugoslavia, both tried to ethnically cleanse each other, both have major territory disputes, both refuse to engage in any relevant diplomacy to this day, and both sabotage each other on countless matters, from industry to sport.
They're both Christian, but one is orthodox while the other is catholic, so calling each other heretic is a pastime for both clergies.
It doesn't help that both Croatian and Serbian are mutually intelligible, so there is no language barrier whatsoever and the notion of being Hrvati or Srbi is purely ethnical and cultural, kinda like the English and Americans.
Few countries in the world hate each other like Hrvatska and Srbija do.