Serbs are Orthodox and the Croats, Catholic.
The Serbs were more kindly disposed towards the Muslims, having been overrun at various times by either Catholics or Muslims, because the Catholics tried to convert them by force *and* impoverished / starved them; but the Muslims wanted them to do well enough that the jizya tax would be worth collecting.
Also, Gavrilo Pricip was a Serbian who killed the Archduke Ferdinand (and started World War 1) because he wanted to join a "Serbian / Yugoslavian" nationalist group called The Black Hand...but they originally wouldn't let him because he was too scrawny (among other things, he had tuberculosis). The assassination was his way of proving, that he "was too, good enough."
What a dick.
That was a war just waiting for a spark. If the Serbian guy hadn’t lit the fuse, something else would have at some point.
We had Serbs and Croats at our high school (Chicago) I was not allowed to hang out or date Croats. There was one I was just friends with we liked to play tennis Dad always gave me the stink eye. He was Orthodox but married my mom a Polish Catholic and she made him promise if they married the kids would be raised Catholic. Dad went to his church and we went to ours.