Serbia regained Kosovo after 400 years of Turkish/Muslim occupation; the Balkan Wars (1912-1913). Ever since 1913 the Albanians have been accusing Serbia of "stealing their land." In WWI, they were allies of Austria & Germany committing unspeakable atrocities against the Serbs. Then came WWII - they allied with the Nazis and Italian Fascists, again committing countless atrocities against the Serbs.
After WWII, the Communists took over and basically allowed the terror to go on, while legally banning the Serbs from going back to their homes and farms (the refugees as well as those Serbs who joined the fight against the Nazis were forbidden to return to Kosovo under "Resolution 355"). That was not enough, so the Communists gave the Albanians autonomy in 1974. The terror, this time institutionalized, continued. The NY Times published a bunch of articles in the late 70s and 80s documenting Albanian atrocities.
The 90s saw another decade of atrocities, attacks on non-Albanian civilians, terrorist attacks on cops, soldiers, state employees, etc.
That's it in a nutshell.