Is it pure coincidence that these two landmark cities, known around the world as fountainheads of the most vibrant and creative aspects of American culture, have become our two direst examples of urban failure and collapse? If so, it’s an awfully strange one. I’m tempted to propose a conspiracy theory: As centers of African-American cultural and political power and engines of a worldwide multiracial pop culture that was egalitarian, hedonistic and anti-authoritarian, these cities posed a psychic threat to the most reactionary and racist strains in American life. I mean the strain represented by Tom Buchanan in “The Great Gatsby” (imagine what he’d have to say about New Orleans jazz) or by the slightly more coded racism of Sean Hannity today. As payback for the worldwide revolution symbolized by hot jazz, Smokey Robinson dancin’ to keep from cryin’ and Eminem trading verses with Rihanna, New Orleans and Detroit had to be punished. Specifically, they had to be isolated, impoverished and almost literally destroyed, so they could be held up as examples of what happens when black people are allowed to govern themselves.