Sometimes the social construction of race can flat out contradict historical and genetic reality. Eastern Europe, past and more recent, will be used as an example. In high school I was always taught that two of the major causes of the First World War were nationalism and alliances. This is also true, to a certain extent, of the Second World War. Nationalism was simply described to me as extreme pride in ones country and a belief that ones country was superior to another. But the notion of being from the superior country, and having that make one superior than someone else from a less superior country also ties into nationalism. But this notion manifested itself into racial superiorities and inferiorities with nationalistic and linguistic overtones. The Russians saw themselves as racially superior to the Hungarians, who saw themselves as racially superior to Serbs, who saw themselves as racially superior to the Bulgarians, who saw themselves as racially superior to the Albanians, who saw themselves as racially superior to the Romanians etc. The fact of the matter is that these people who live in Eastern Europe are all Slavic.
Slavic people are speakers of the Slavic language family, which is branch of the Indo-European peoples living mainly in Europe. And the Slavic homeland is Eastern Europe. In the sixth century some migrated to Northern Asia (Russia, etc.) and into the Balkan region. And there are some subdivisions within the Slavic people. Western Slavic refers to Czechs, Poles, and Slovaks. Eastern Slavic refers to Belarusians, Russians, and Ukrainians. And Southern Slavic refers to Bosnians, Bulgarians, Croats, Macedonians, Montenegrins, Serbs and Slovenians. This shows that the social construction of race can be inaccurate when measured against historical and genetic evidence.
The Croats and Serbs have committed genocides and ethnic cleansings for decades. There have been such atrocities even before most of the countries declared their sovereignty from the former Yugoslavia. The violence witnessed between the two during World War II did not occur because genocide is a "natural" product of Serbo-Croatian relations, but rather because of the desire of the fascist Ustase to create an ethnically pure Croat state, which in turn called for the elimination of Serbian and Muslim minorities in Croatia.
Radovan Karadzic was in fact a family doctor who practiced on people regardless of ethnicity until he became the infamous president of the Republika Srpska. What occurred then was a result of manipulative leaders like Ante Pavlic, Slobodan Milosevic, and Franjo Tudjman who exploited and manipulated people's nationalist leanings to gain power and prominence. While it is true that the people must go along with them for their agendas to be successful, it is also true that without such personalities, the potential for violence in the Balkans is significantly diminished.