Well, the civilian ships visited the system several times. It turns out I misunderstood what I was seeing.
I thought the Sheridans and the Ivanovas were both native to that system. It turns out that the Sheridans are from the next system over. What I thought was the Sheridan homeworld was the Ivanova homeworld, occupied by the Sheridans. What I thought was the Ivanova homeworld was just some minor colony on a habitable world.
So I thought I was coming in to conquer two small races who'd beaten each other senseless, rendering them both easy pickings. Turns out I instead stole a meal from a predator of unknown size and I've got an actual fight on my hands.
This is pretty awesome. I love the limited information.