1930s: In 1935, Harlem erupts in a riot when an African-American mob falsely believes that an African-American boy has been beaten or killed by a white storeowner.

1960s: In 1964, Harlem erupts in a riot when a black schoolboy is shot and killed by a white police officer in Yorkville, New York. The incident inspires riots in one other New York city and three New Jersey cities. More than two hundred other race-related riots break out from 1965 to 1968 in various cities around the United States, including Detroit and Los Angeles.

Today: In 1992 in Los Angeles, a bystander catches an apparent act of police brutality on videotape, which he then sells to the press. In the resulting court case, a mainly white jury acquits the four white police officers of the beating of Rodney King, an African-American motorist. This verdict enrages many ethnic groups in South Central...