The Barbarians — Nathan Hale

Fifty-six years earlier Earth was transformed from a bustling paradise with more than seven billion inhabitants into a shattered wreck with fewer than four billion in less than two hours. Virtually every military installation, power plant, dam, and city on the planet was struck by the kinetic energy weapons fired from space by the various combatants leaving behind only a huge hole up to fifteen hundred feet deep and as much as forty miles wide. Then the real damage began.
The worlds coastlines were quickly swept bare by tsunami after tsunami which was caused by the KEW's that were used to sink the world's navies. The massive waves also swept up the worlds rivers leaving behind only more death and destruction. Within another two hours at least another billion people died. Then, because of the huge volume of debris thrown into the upper atmosphere the unending worldwide winter began. By the time it ended seven years later roughly five hundred million people had survived only to find themselves struggling in a world with very limited resources.
Only the Association did survive and finally, after fifty-six years, they are able to send out an expedition to locate other survivors while observing the extent of the continuing destruction that the Great War had caused. With this in mind the Board authorized General Ramirez to put together a small exploration party so they could go and see if such groups existed. A month or so later Captain Goldman left the Association with said party which consisted of about two hundred forty officers and men, four-three inch ordinance rifles, more than a hundred large cargo wagons plus remounts and even a small group of scientist's.
What the captain and his men discover is that the world around the Associations little garden spot is a very dangerous place on every level. As the world seems to fall apart around them they find friends and an implacable enemy whose sole purpose in this life is to completely destroy all of the demons that had injured their God, Mother Gaia, so grievously.
The expedition quickly changes from one of exploration into one of sheer survival. They must survive to return to the Association so they can warn about what is coming. Only the followers of Mother Gaia and her son, the Lesser God Epa, have their own agenda. As they have done everywhere they've gone they will first kill this group of demons and then they will kill all of the demons where these came from.