It would be helpful if you could tell us which historical period you are asking about. I am guessing that you are asking about the time soon before WWI. During this time, the growth of nationalism made the Balkans a "powder keg" that would blow up and help cause WWI.
In the time just before WWI, the Balkans were mainly part of the Austro-Hungarian Empire. The rulers of this empire were ethnic Germans while most in the Balkans were ethnic slavs. For this reason, the Slavs were unhappy as part of the empire. Their attempts to form nationalist resistance movements caused a great deal of trouble. This culminated with a nationalist killing the Archduke and starting WWI.