Hi!
I keep seeing other civs make peace with them;
That should be a game glitch. Nobody besides Korath should be able to make peace with them. However there's a solution a player reported: when he got a message someone made peace with DL, he CLICKED that message and was transferred to the diplo screen and was able to talk with the DLs. You can try that in your game.
If that will not work, you have two options: either strengthen your planets DLs will not be able to take them by invasion, or deliberately allow them to take planets, but immediately take them back, with optional big gain in tech from them. In the later case DON'T RESEARCH any logistic tech, or you'll soon face FLEETS of DLs' ships.
In either case you'll need to research:
- all of ground combat techs, with Tir-quan training,
- all growth-boosting tech and wonders and
- all morale techs and wonders
in order to lessen the losses you'll have with invasions, and grow back pop ASAP. It will also help if you build a farm on every planet you dont' want to "give" to DLs.
To deal with DLs' ships you'll need Eyes of the Universe "wonder" and best hulls with best weapons, one-two engines and no defenses. Keep hunting their smaller ships first, in order to remove their "eyes". Only pick batteles you can, even with losses. Your economy will quickly replace them. If it can't, better invade your neighbours to make your economy bigger.
Also cover all DLs' planets and paths to your planets with your "eyes": fast and cheap ships (cargo or small hull with a single laser-5, 2+engines, no sensors - Eyes will make them see 15 squares), that will also serve as interceptors for their transports.
Good hunting!
BR, Iztok