The former Yugoslavia-land of mass graves. Does anybody like anybody over there??
The thing is that outside powers have always supported the non-Serbs in the wars, and now the U.S. is the current torch-carrier in all this: supporting the Croats and Balkans Muslims (and other separatists) against the Serbs.
Former major powers/players - the Ottomans, Austria-Hungary, Germans, and more - have wielded "divide and conquer" strategies on the Balkans peoples for centuries, and so by Europe and the U.S. supporting separatists, starting around 1990, things fractured around old fault lines.
And the "reconciliation" talk and policies of the international community involvement today is very fake - there's another agenda going on then their claims of being for multiethnic and multicultural communities. The peoples came together after the bigger wars of WWI and WWII much, much better and more natural than what is happening now with continued occupations and involvement by NATO.
For much of the post-WWII years of the 20th century, the peoples of Yugoslavia lived and worked together, not in perfect harmony, of course, but strong enough to create an identity in which many were proud. Tito gave the new Yugoslavia its identity: one that could say "No" to Stalin, while receiving the favors of the US. It was the time of the building of the modern Yugoslavia and ethnic tensions could be controlled and sometimes transformed in the great work of rebuilding and creating. The mass graves generally are found after war. During times of relative peace, people may simply "disappear," only rarely to be found, as in the Central and South American countries (too numerous to list) that killed its own citizens during the 1980's. There you could look for mass graves and ask the same question. War unleashes horrifying atrocities; ethnic conflict can be worse but not always: see the American Civil War.