Many of these efforts have failed and few have succeeded. But what is truly painful is how often the polarization created by these efforts led to tragedies, such as civil war in Sri Lanka and brutal mass expulsions of millions of Germans from Czechoslovakia, to the detriment of both the Germans and the Czech economy.
The history of blacks in the United States has been more complicated. By the end of the 19th century, the small numbers of blacks living in northern cities had, over the generations, assimilated the culture of the surrounding society to the point where they lived and worked among the white population more fully than they would in most of the 20th century.
In New York, Washington, Chicago, Philadelphia and other Northern cities, black ghettos became a 20th century phenomenon. It was after the massive migration of far less acculturated blacks out of the South in the early 20th century when a massive retrogression in black-white relations took place in the Northern cites to which the migrants moved.
The blacks who moved to these cities were of the same race as those who were already there, but they were not the same in their culture, values and behavior. No one complained of this more bitterly than the blacks already living in these cities, who saw the newcomers as harbingers of a worse life for all blacks.
This same process occurred on the west coast decades later, largely during World War II, when the same influx of less acculturated blacks from the South marked a retrogression in race relations in places like San Francisco and Portland.
Cultural differences matter. They have always mattered, however much that may be denied today by the multicultural cult.