Countries that basically have one ethnic group include

The Socialist Republic of Bosnia and Herzegovina issued a referendum for independence in 1992. Bosnian Serbs, who had been against the referendum boycotted it and formed their own separate country. After Bosnia and Herzegovina declared independence, the Serbian president, Milosevic mobilized his troops to seize Serb territory. War broke out accompanied by the attempted ethnic cleansing of the Muslim Bosniak and Croat population

Connecticut, delaware, maryland, massachusetts, new jersey, new york, new hampshire, north carolina, pennsylvania, rhode island, south carolina, virginia

They immigrated for religious freedom