[LorenPechtel]

Disagree, Kzinti Lesson.

It's pretty much impossible to come up with a stardrive that doesn't have a weapon potential that makes nukes look like firecrackers.

1) The direct Kzinti Lesson: Anything that can push a ship to relativistic velocity produces incredibly energetic exhaust. The efficiency of the drive is directly related to how well you can point it in one direction, thus making it very hard to make something that isn't a weapon. And in a population-attack scenario such a starship throwing rocks is devastating. At 86% of lightspeed a rock hits with it's annihilation energy. At 94% of lightspeed it hits as hard as if it were antimatter.

2) Jump drives (whatever you call them) still need to get somewhere without waiting years. While it doesn't have nearly the planet-killing potential of a relativistic starship it's still quite capable of throwing city-killers.

3) Intertialess (Lensman series). No direct weapon potential but they knew they couldn't crack the defenses of Jarvenon--so the slapped inertialess drives to a couple of planets and used them as a nutcracker.

Known physics only leaves the first scenario as possible, I threw out the others to cover the various sci-fi drives. I can't think of any story that uses something that doesn't fit one of these three categories.