They landed, and you met face to face. They were strange, strange creatures. the closest you would call them was cephalopods turned anthropomorphic. You couldn't really handle how strange they were, but the leader and diplomats persevered anyways. You idly note how some Earthborn would probably have found them mysterious and would've probably considered Xenophilia.
They spoke of massive factories, even bigger ships. all of them automated, all of them dormant, waiting for a human touch. an aliens couldn't do as Mans tools were meant for man alone at the time. These would be a start, as such tools would most likely allow the colony to survive whatever minor fleets it would come by to exterminate them again. Mankind on the colony had experienced a population boom, having reached a full two million. Synthetic workers were rapidly becoming commonplace. Your workforce was rapidly divided between highly trained soldiers, trained from birth, or scientists, engineers, and men of the mind. Manual labor was done by criminals, the forgotten and the synthetics, mainly synthetics. It was easier to build a machine then to raise a person for the duty, most of the time.
A. The first skirmishes were done with humanities finest in front, each killing a horde before falling.
B. Massive synthetic war machines dominated the battlefield, before their ravenous barrels destroying entire regiments before falling.
C. Mankind's legacy was realized. a Massive dreadnaught had been awoken, and its assistance had resulted in several successes in the battles among the stars.
D. Another empire, with a agenda of its own, saw humanity and offered its assistance in return for help in cracking the artifacts of Humanities legacy.
((I googled it, nice.))