How did the growth of the nationalism affect the Balkans?

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.