The long excerpt on the website goes into it. In short, all of the major bridges over the Ohio, Tennessee and Mississippi Rivers in the region collapse. The area between Nashville, St. Louis and Little Rock is devastated, but Memphis and Western Tennessee are the worst off.
At the start of the novel, it's one year after the two New Madrid earthquakes. Battalions of foreign peackeepers are occupying Tennessee, at the invitation of the President.
Phil Carson, (from "Enemies Foreign And Domestic"), and three strangers are hiding in a well-stocked cave, which is a guerrilla fighter's lair. Across the region Kazakh "contract peacekeepers" are wiping out the last remaining American holdouts, who have rejected the federal government's order to abandon their homes and move to "relocation centers."
I read it, that’s why I pinged you to this.