Compared to other countries, America seems to have a great amount of racial tension. Many attribute this to slavery, however the fact remains that 95% of slaves in the western hemisphere went to other countries besides the US-- and that's only in the WESTERN hemisphere, while a good deal went to other nations as well.
Today, those nations have nowhere near the level of racial tension, resentment, hostility and animosity of the US; riots, "Civil Rights" protests, and other general vitriol seem almost non-existent in many of them.
The cause of this difference, is definitely the Civil War, whereby the mass-invasion and violation of state sovereignty was accompanied by false claims of emancipation-rights-- as well as deliberate race-baiting agitation and incitement of hatred and resentment where none existed previously, via totalitarian means; likewise, propaganda and blame-- such as the legality of the Civil War and forced emancipation and the evil racism of the south-- were inaccurately spewed, while factual information such as Yankee racism and general southern integration and tolerance, was suppressed.
The devastation of the South's economic and poltical infrastructure, which continues to this day, created a general racial hostility which resulted in a displacement of a large part of the African-american population into Yankee territory- where reality regarding supposed Yankee "tolerance" was learned first-hand via the formation of black ghetto's due to northern segregation-methods, such as discrimination in both the public and private sector (police trumping up charges against blacks, job and housing discrimination, etc).
This naturally resulted in hostility, including oppression and riots, which naturally followed such injustice and disparity between lip-service to freedom, and the actual treatment received.
Hence, the popular lie that "racism comes from the south's evil slavery," is probably the biggest lie of all.