Kosovo is similar to an Israel-Palestine situation, you could trace the original settlement to one group but the other group isn't going to simply pack up and leave. The two ethnic groups in the area have always been united against some common enemy before, some of those enemies being the Turks, Facists, Capitalist countries, etc. After the USSR breakup and the dissolution of Communism in Europe as a whole the groups have no 'enemy entity' to unite against and nationalistic fervor took over and individual groups became more self-determined.