Their government has moderated in recent years since the nationalists were ousted in free and fair elections and they're much more pleasant now. During the nationalist years, they first fought on the side of the Bosniaks (Bosnian Muslims, basically), then against the Bosniaks (and actually
lost that war), then on their side again. Sometimes they did both at once. The Serbs would shell whichever side was stronger during the Croat-Bosniak scraps. Weird-ass war.
Also, despite European-UN pressure and condemnation, their successful 1995 assaults on Croatian Serb areas (and, then, on Bosnian Serb areas in conjunction with the Bosniaks) did, basically, lead to the end of the "Wars of Yugoslav Devolution", in contradiction to a lot of European establishment opinion which said only giving in to every Serb demand would bring peace. Idiots.