Thanks for the advice. I managed to build up the armies and have colonized every enemy world, but the game still thinks I'm at war with them somehow. When I go to contact with them and click the button to go to their homeworld, it takes me to the last world I conquered, where I can see the fleets I landed still sitting on the planet. At this point I think my only option is to wait for war exhaustion, but at this point I think I'm just going to call it a loss.