Guardian Directive

Primitive humans on Earth have been eking out an existence as hunter-gatherers and farmers for millennia. Some live in large villages and small towns, while others haven't yet discovered the wheel. What could be a worse time for an alien invasion? But by a beneficial turn of fate, an altruistic race of space explorers discovers the coming invasion before it starts. Only twenty-five years earlier, they too had discovered the Earth and had been unobtrusively studying the local inhabitants. Although unprepared to stop an invasion, they move to intervene, hoping to save the innocents on Earth from domination and potential annihilation by a ravenous species of conquerors.
The wormlike Rhulle want to colonize the rich planet as a springboard for further conquests, and they plan to cultivate the indigenous humans as a food source. The far-thinking Hasdradan want to protect Earth and allow its primitive humans to evolve into potential trading partners. The Rhulle and the Oligarchy of Hasdrada have been fighting a bitter war of attrition and annihilation for eighty years. Now, the latest battleground turns out to be Earth, in the year 4350 BC.