The Three Armies
The Survivor splinter group was overcome with agoraphobia. They took to their shovels and went back underground. And they tunneled. They dug quite a distance; they don't really know how far, but even after 40 years, they're still digging.
They went over their battle plans to find out why the Evolved came so close to beating them, and they hit the drawing boards for new vehicle and weapon designs.
They think they've got it right this time, and so it's time to break on through to the other side and smack some freaks up.
The Evolved splinter group took to their bare heels and scarpered across the land. They slowed down eventually, but their nomadic tendencies kept them traveling, and gave them a lot of time to think.
They figured that the reason they had had quite a hefty mutated butt-kicking was that they had stooped to the ways of the old humans - they had used vehicles and other assorted technologies, and the Gods had frowned upon them.
They decided to rebuild their war parties using only mutated animals and warriors.
After 40 years, they're ready for anything. And they're about to have to deal with everything.
The Series 9
Of the nine series of robotsz, created to take over all the menial tasks of humans, only one survived the great Nuclear War of 2079.
Most robots (Series 1-4) were designed for city use, and since the cities were all targeted by warheads, well kaboom! The Series 9, agricultural robots, were far enough from the cities to survive the blasts.
Sure, the radiation and shockwaves freaked them out a little and they personally wiped out Series 5-8, but hey, everyone can go a little billy every now and again, right?
Anyway, it took them many years to develop an understanding of what had happened (the humans had destroyed their crops and their soil - and hence their entire reason for existing), and took them more years still to develop a new emotion - Anger.
In 2140, the year of the Great Surface War, they showed up, finally, to punish the humans, only to find that the humans had plasma cannons and giant acid-spitting scorpions, and all they had were forks and shovels.
So they hit their own drawing board, using ideas from the battles they had witnessed.
It's now 2179, a hundred years since they were rendered bored. They've got the motive, they've got the weapons, and they've got no respect for human life. At all. Time to crash a party.