Why did ethnic tension become such a severe problem in the soviet union and yugoslovia

Ethnic tensions were a problem in the USSR as well as Yugoslavia because there were certain ethnic groups such as the Croats and Bosnians in Yugoslavia that felt they did not belong in a unified state where they did not share a common religon, a common language and a nationality. You can't blend a bunch of people that share nothing in common in one country. The tensions are bad because some ethnicities see others as threats to their survival in the nation and use unethical methods such as terrorism and genocide to get their way. Serbians killed Bosnian Muslims for example.

abraham lincoln  (february 12, 1809  – april 15, 1865) was an american statesman, politician, and lawyer who served as the 16th  president of the united states  from 1861 until  his assassination  in april 1865. lincoln led the nation through the  american civil war, its bloodiest war and its greatest moral, constitutional, and political crisis.[2][3]  he preserved the  union,  abolished slavery, strengthened the  federal government, and modernized  the u.s. economy.