This is just the tail end of conflicts ranging from the first Slavic movements (Later Roman EMpire), into the Balkans to the breakup of Tito's Yugoslavia. following this strong man's death. At one time, the Serbs controlled an empire ((1100-1400's, under their king, Stefan Dusan and his successors) They centered their empire in what is now known as Kosovo. Hence the spiritual and historical connection that the Serbs feel for the province.
Following the Ottoman victory at Kosovo Pole, the Serbian kingdom was extinguished. To humiliate the Serbs and remind them that they were now under Muslim rule, the Ottomans settled large numbers of Albanians, recent converts to Islam, into the province. Serbs seethed with resentment and bided their time.
When the Ottomans retreated from the Balkans in the 1800's, they left these problems to the Austrians who took their place. The Austrians divided the Balkans into managable regions (which is why you have Montenegrins, Bosnians, Macedonians, Croats and Serbs), each agitated for autonomy, but the Serbs were the most vocal, culminating in the assassination of the Archduke Ferdinand, which brought on WWI.
The Kingdom of Yugoslavia (established by the Treaty of Versailles) sought to alleviate these regional conflicts, by trying to reestablish a one state under a monarchy drawn from one of the European families. This worked superficially until the German invasion and Occupation of 41. Yugoslav resistance to the Fascist occupation was led by the old monarchists (Chetniks) and the Communists,led by Josep Broz (Tito). Tito, a Croat, looked to a greater Yugoslavia, devoid of tribal distinctions, held together by one embracing communist party. Mihailovich, the Chetnik ,wanted a return to the monarchy. Both fought with each other as much as they fought the Italians and Germans. To complicate things, the Germans raised units from the Balkan Volkesduetches and the Albanians to combat the two. The Albanians and Bosnians, both considered ethnic and religious minorities, threw their lot in with the Fascists, in an effort to regain their old control.
With the Communist victory in 1945, Tito, having rid himself of his rivals, established the Greater state of Yugoslavia. Despite being a Croat, Tito recognized that there was only one state and one people. He placed Yugoslavia in a "neutral" zone between East and west, taking aid from both.
He was a constant pain in the side to the Russian leaders from Stalin to Breshnev.
Upon his death, Yugoslavia broke up again along tribal and religious lines. The state needed a strong man, and his son was not of the same caliber. The Serbs, being one of the larger, reasserted its claim over the other Slavs.
The Serbs retook Kosovo, attempting to expel what it considered the invaders on Serbian holy ground. Then, it tried to retake what had once been the entirety of Yugoslavia. Though being armed the best, the Serbs could not accomplish their grand design. The West aided the Croats with masssive military shipments. However the most disturbing was that the Moslem states of the Middle east, sent in "volunteers" to their co-religionists, as well as arms. The Serbs, barely held on to their regions, and there just weren't enough Serbs to resettle Kosovo.
Sorry for the winded lecture, but this is just a very brief synopsis. To the Serbs, this is their birthplace, their kingdom (wrested from a Failing Byzantine empire) and lost in the Ottoman victory of Kosovo Pole (which decimated the Serbain nobility). It would be as if an invader took the Carolinas, we if would fight to regain our land.
We would be insulted if some other countries told us to get over it and recognized the occupation. Heck, we told GB qnd France to bug out of our civil war.