Nine million is considerably high, it's not much lower than the base population of Oregon and Washington combined together. Claiming it to be so high crudely brushes away the stresses that'd be caused from nuclear war. Food supplies would dry up fast and the former medical infrastructure would go the same way. You'd have suffered severe shortages resulting in hunger or even medication or vaccinations to fight otherwise curable diseases.

Post war too, fuel supplies would have been dried up even faster so even cars and farm tractors would stop working. That too doesn't stress the long-term issues that would complicate running even a simple farm-tractor. They and cars might have a reputation as coming off as being very simple machines, but they are really rather complicated.