How about this:
In a star system about 80ly from Earth (give or take a few parsecs) there is a nasty, reptilian alien race. Humanoid--not all that dissimilar to us, actually. Some probes swept by Earth early in the 20th century. One even crashed near Roswell, NM. But the other one flashed back data that showed an inhabited planet, with large liquid oceans.
So the aliens used their two-pronged approach to colonization. They have perfected fusion drive, capable of accelerating small masses (say, the size of Volkswagens to the size of a house) to around 0.8c. Larger masses (i.e., a colony ship) require so much fuel to accelerate that the best speed practicable is more like 0.4c.
So what they do is this: they send about eight small ships at 0.8c carrying large, high-yield but clean thermonuclear devices with yields of around 250Mt. They arrive first, detonating high above the atmosphere, sending out a rather potent EMP burst. A sort of "Civilization-B-Gone" extermination tactic, knocking out any high-energy civilization on the planet to be colonized, yet causing little physical damage (they want a working, inhabitable planet, not a mere cinder of a resource node) beyond flash burns and some scorching and irradiation of vegetation near ground zero for each of the bombs.
Then, a couple centuries later, as the victims are still living in the ruins, perhaps taking the first steps to recovery (like rediscovering electricity), the colony ships arrive, with thousands of colonists and soldiers, to stake out a small area of the planet to build on...and the next stage of conquest begins.