The importance of the Appalachian defenses could not be overstated. With the loss of the coastal plains, and much of the Great Plains, the sole remaining large areas for food production were Central Canada, the Cumberland plateau and the Ohio Valley. And although the Canadian plains were high quality grain production areas, their total production per acre was low and they were effectively unable to produce a range of products. In addition, while there was increasing industry throughout British Columbia and Quebec, the logistical problems of a broad-based economy in nearly sub-Arctic conditions that had always plagued Canada continued even in the face of the Posleen threat. It was impossible to shoehorn the entire surviving population of the U.S. into Canada and if they did the survivors would be no better off than the Indians huddling in the Gujarrat and Himalayas.
Lose the Cumberland and Ohio and that would be for all practical purposes the end of active defense. There would be humans left on the continent, but like all the other major continents, they would be shattered survivors digging for scraps in the ruins.
Knowing that the lower Great Plains were indefensible the forces there, mostly armor and Galactic armored suits, had retreated, never engaging unless they could inflict terrific casualties. This retreat had ended near the Minnesota River for much the same reason as the Siberian retreat. However, the Posleen had succeeded in one objective, whether they knew it was an objective or not. In the long withdrawal, the 11th MI, the largest block of GalTech Armored Combat Suits on Earth, was destroyed.
All of these defenses were predicated on the Posleen’s major weaknesses: inability to handle artillery and inability to cross significant barriers.