Your encounter a human slower-than-light ion-drive starship in deep space.
They haven't developed faster-than-light drives or reactionless thrusters! Instead, they use ion drives, beamed-energy solar sails, and mass drivers for in-system transport at their home system.
Your ship's telepath reports that they're completely non-violent and pacifist, to the point where they don't even have a concept of "war" and medicate anyone with violent or anti-social tendencies into conformity.
You send a report back to your empire, which will of course proceed to assemble a conquest fleet -- this will be a curb-stomp battle.
Next you fire a heat-ray induction weapon at their ship to force them to surrender.
One of the humans goes off his meds and proceeds to slice your ship to pieces with his ship's ion drive exhaust.
Your battlefleet reaches the Solar System, knowing they're facing an enemy incapable of defending themselves, and without weapons if they tried...
Except for the lasers used to drive solar-sail spacecraft.
And the ion drive Belter ships.
And the mass drivers.
And what they learned from the wreckage of your ship.
Curb-stomp battle... just not the one you expected.