Bosnia and Herzegovina
The state is home to three ethnic/religious groups who share a long and complicated history of conflict, accommodation and sometimes peaceful co-existence: the Muslim Bosniaks, the largely Catholic Croats, and the mostly Orthodox Serbs.
Bosnia remains the case study for the evils of unchecked nationalism. The assassination of Franz Ferdinand, which triggered World War I, took place in the capital city of Sarajevo. The assassination was purportedly plotted by the Black Hand, a Serbian nationalist movement aiming to unite all Serbs—including the ones living in Bosnia and Herzegovina—under a single flag.
The 1990s were a particularly bad time for the country, as it experienced several years of nasty fighting (which was a civil war or a war of foreign aggression, depending who you're talking to) which led to mass atrocities and the events that led to the world's first criminal conviction for the crime of genocide at Srebrenica. The one consolation was that the Siege of Sarajevo introduced the world to Vedran Smailović, the famed cellist who played in destroyed buildings and funerals (the latter being a favorite target of snipers). Smailović evidently couldn't get enough of politically fraught places, and upon escaping Sarajevo in late 1993 went to live in Northern Ireland.