How has ethnic diversity and nationalism within the Balkan region (specifically Serbia, Kosovo, Bosnia, Croatia, and Montenegro) led to conflict?

As much as any country can be an artificial construct forced together—despite serious built-in fissures forcing its collective parts apart—Yugoslavia was such an entity. States, in the international sense of the word, are political entities within the borders of which can coexist numerous ethnicities and religions. Coexistence, though, can be tenuous, and Yugoslavia was doomed to disintegrate.
Yugoslavia was conceptualized during the eighteenth-century as a way of bringing together into one state the whole of the so-called “Southern Slavs,” particularly Serbs, Croats, and Slovenians. The state of Yugoslavia was not actually established, however, until the end of World War I, when the destruction of the old European empires and the descent into absolute chaos of Russia allowed for the creation of newly-independent states, including Czechoslovakia (which merged the Czech and Slovak nations), as well as Yugoslavia, which, in addition to Serbs, Croats, and Slovenians, also included Macedonia, Bosnia-Herzegovina, and Montenegro. In addition, the Kosovo, an overwhelmingly Muslim region, was incorporated into Serbia, ensuring future instability there. The longest serving leader of Yugoslavia was Josip Broz Tito, a half-Croat, half-Slovene communist revolutionary who led an anti-German partisan movement during World War II. With the Soviet Union’s victory in the Eastern Front, Joseph Stalin was able to influence the establishment of governments throughout Eastern Europe, including in Yugoslavia.
Despite Stalin’s assistance in establishing a communist government in post-war Yugoslavia, Tito broke away from the Soviet Union’s orbit in 1948 and ruled Yugoslavia as a non-aligned nation until his death in 1980. However, throughout Tito’s long reign as general-secretary of the Yugoslav Communist Party and president of Yugoslavia, it had long been anticipated that, with the eventual death of this formidable dictator, the country that he held together through a carefully-calculated combination of force and conciliation would eventually disintegrate—and it did. With Tito’s death, no other single leader was capable of holding the vastly disparate parts together. Soon-enough, Roman Catholic Croatia sought to distance itself from Eastern Orthodox Serbia, which was finding it increasingly difficult to maintain its hold on Muslim Kosovo. Unfortunately, for all citizens of Yugoslavia, Serbian leader Slobodan Milosevic and Croat leader Franjo Tudjman were both determined to seize onto as much of Bosnia-Herzegovina, a predominately Islamic region with minority Serb and Croat populations within its borders, as possible. The multi-faceted war that broke out in the early 1990s, then, was a reminder of the tenacity of ancient ethnic and religious divisions. The war was also a reminder that the barbarity of World War II had not been eliminated from post-war Europe.
In short, Tito’s inevitable death was, as expected, the catalyst for the disintegration of the state he had ruled for so many years. The divisions that existed within Yugoslavia between World War I and Tito’s death in 1980 had not been eliminated; they continued to exist but had been held in check by the force of Tito’s personality and the legitimacy he held in the eyes of much of the state’s populace due to his record as a partisan and a communist party leader willing to distance himself from Joseph Stalin, one of the bloodiest rulers in human history.