We have been reading W.E.B. Du Bois' The Souls of Black Folk in our social theory class. Du Bois famously said, at the dawn of the previous century, "the problem of the Twentieth Century is the problem of the color line.”
I had not fully realized until this reading that he did not just mean that the color line is the main problem of the twentieth century in the United States, but rather, that this is the global problem of the century.
Later in the book he writes “the characteristic of our age is the contact of European civilization with the world's undeveloped peoples.” When we think about the world in 1900, almost the entire globe was directly ruled by European countries or their colonial heirs. The Europeans and their transplants operated on an explicitly racial theory which held that the white race(s) developed the world. The "undeveloped" peoples were those on the other side of the color line globally.
In the second half of the twentieth century Du Bois' prophecy came true with striking clarity all over the world. We are still working through the aftermath of ending racist and imperialist theories that justified European domination. The problem of the color line is far from over in the global clash of civilizations. But the theory that justified the color line has been subverted.