Repleeta...
I don't know the dates, but what happened was this: Terran scientists on Repleeta, on learning of the war, went over to the Kilrathi scientific outpost on the other side of the planet, to give a gesture of goodwill. Their governments may be at war, but they were scientists...they were not.
The Kilrathi, of course, slaughtered the idiots.
Confed sent marines, and the Kilrathi did so. Though a single space strike could have resulted in victory for either side, Repleeta came to represent what each side could accomplish with little or no reinforcements.
Eventually (about 20 years later), a lieutenant (Miles D'Arby, I think) led the remaining Confed marines in a last-ditch attack. Both sides were wiped out.
LOAF can correct me with the dates, but I don't think there's any kill count for either side...