I'll bet most Americans don't know about the travails of Macedonia. This article in the Wall Street Journal does a nice job of laying out the conflicts between Greece and the country that would like to call itself the Republic of Macedonia, but sometimes calls itself F.Y.R.O.M. or the Former Yugoslav Republic of Macedonia.
As the article shows, the conflicts between the two countries deal with ethnic conflict, language conflict, religious conflict, and political conflict. There is a history of ethnic cleansing, issues of right of return of driven out former residents, and concerns about eventual Macedonian claims on the territory of northern Greece. And the article doesn't even mention that Macedonia is being overrun by Albanian economic refugees.
The problems here are classical Balkan problems: a long history of inter-ethnic conflict and bloodletting, an absolute refusal to forgive and forget prior wrongs, and the bitterness is extreme because the stakes are so low (hat tip to Columbia University political-science professor, Wallace S. Sayre, who used this formulation to describe academic politics).
