[Baal]

There was some mention of "pre-space faring" races on planets. Take it a couple steps further.
-Be able to colonize the planet or invade it and use the indigenous population (nothing new)
-Be able to attack an enemy empire and "bomb them back to the stone age". Take out all their facilities, weapon platforms, sources of technology thus turning them into a "pre-spacefaring" race. They would retain the population.
-Accomplish this by making normal weapons not hurt populations except for a little collateral damage. This forces planetary bombardment research.
-Allow for "pre-space faring" races assent to space faring empires. So, you can bomb them back to the stone age but they can again research space flight and turn into a new empire (not related to the original).