To address Stephan Jaeckel’s point, Detroit faced economic challenges immediately following WWII. First, auto plants were looking for more space and saw the suburbs as the place for plant expansion. That later translated into plants in the South and eventually across the world. Second, the city was overcrowded and expensive and that fueled suburban growth. So yes, the seeds of its demise came much earlier than most people think. Check this out:

http://cornersideyard.blogspot.com/2012/02/reasons-behind-detroits-decline.html