Centered around a series of natural disasters of biblical proportions that are occurring on a global scale, tens of millions have died as a result of the earthquakes, floods, hurricanes, gigantic super cell thunderstorms and tornados that are ravaging the earth.
A rogue nation decides to take advantage of the chaos at this time to attack America with nuclear weapons. The American intelligence network learns of the plan and the government decides to launch a first strike at this rogue nation. This information leaks out to other nations and soon the world is poised on the brink of all out nuclear conflagration as the nuclear nations choose sides in the coming conflict.
As the militaries of the world go underground in preparation for the attack, most of the world is blissfully ignorant of what is about to happen. There are a few exceptions to this. The civilians that are aware make plans to try to survive the attack as well. As the moment for the attack draws near, the disaster plagued planet is beset by earthquakes of unprecedented intensity and numbers. Oddly the epicenters seem to be located at every major military installation around the world. (It is implied and hinted at that divine intervention has saved mankind from nuclear war, though it is never stated outright.)
When the survivors of the earthquakes look around them the next morning they see a world drastically changed. A few men and women struggle to cope with the devastation as best they can but many turn to marauding as a means of survival. Preying on any people they encounter they slaughter any that oppose them as they take anything they want. A few of the survivors band together to protect those weaker than themselves.