In the distant past, Earth was conquered by a cruel race of alien taskmasters from the Omega Sector, who enslaved the primitive earthlings as they had thousands of worlds.
These aliens controlled enormous machines that the earthlings worshipped as gods and to honor them huge monuments and altars of stone were built at Stonehenge, Crete, Pompeii, Angkor Wat, Beijing, Uruk, Cusco, and along the Nile, Ganges and Mississippi Rivers.
Confident in their ability to awe the primitive locals, their greatest war leaders were given their own mechs to control, to fight on behalf of the taskmasters both on Earth and in space, as they expanded ever outward.
These war leaders came to realize how cruel and genocidal their "gods" were and rose up against them, throwing off the yoke of the taskmasters' oppression.
Returning to Earth, they ruled in the Taskmasters' place, not always wisely or well and sometimes fighting with each other or turning into tyrants every bit as cruel as the taskmasters they had served.
Eventually their mortal selves past and the great war machines faded into memory. Those who had never seen them first hand interpreted the heroes through their understanding of the universe. The Omega Sector Taskmasters became gods, their war leaders became heroes like Aeneas, Arthur or Gilgamesh.
In 600 BCE a group of Chaldean astronomer-priests in South America saw the signs in the sky they had been waiting for: the Taskmasters were returning to Earth.
Ball games were held to find the greatest, bravest heroes, worthy of the weapons of the gods. When the invasion was met in orbit around Earth, these heroes found themselves joined by other groups of diligent sky-watchers from Britain and Egypt and the three were able to throw the force, which was really just a scouting party to determine how the insignificant little planet, which was still a part of the Omega Empire by ancient claim, had progressed over the millennia.
In 2012, the Omega Taskmasters have returned again. Not with a minor scouting force but one large enough to return the primitive backwater to their lawful control.
As Earth's military forces are defeated with ridiculous ease, the Chaldeans and the Magi work quietly, recruiting groups of idealistic young heroes to take up the weapons of their ancestors.
The last free people on Earth will either save humanity or doom it to an eternity of servitude.