The Southeastern United States was hit hard by chemical weapons launched from submarines. It's now filled with night dark toxic swamps, and filled with all manner of mutated life. Huge lakes of acid are known to exist around the ruins of New Orleans.
There is no official Census Bureau definition of the southeastern United States. However, the Association of American Geographers defines the southeastern United States as Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, Tennessee, Virginia, and West Virginia.