The 1990's Yugoslavian war where they invaded Croatia and Bosnia because some of the Yugoslav republics wanted to separate. Also all of the ethnic cleansing done by the Serbs to the Croats and Muslims. They have much hatred in them.
that was becaue some muslims shot and killed serbian bride and groom adn then the conflict erupted, also it was pretty much only muslims. The same ones that have taken Kosovo and taking Macedonia.