This is a pretty difficult question, and without knowing about the other stats/setup/version of your game, I might propose something that could probably not viable to you.
Without any real Military the only answer can be to get out of war ASAP. Built up your planets to max pop, get some good soldiering and harvest empty hulls for a final upgrade.
Usually if there are stronger AI's present in a game then it is always the best to let them fight it out until their strengths is drained. Maybe the arrivial of the DL is your advantage. If they could battle against the Drath this would give you the time to built up your empire/military.
However, you need to pay close attention which one of them will be winning - and once one side is knocked out jump into the war and take that weak AI over.
In case that the DL survive and you won't be able to destroy their Defenders simply max out your planetary pop, and focus on getting a strong soldiering. Usually then the DL won't be able to invade your worlds even if they do have space superiority.
And once you have most planets under your control this should give you enough research potential to get to the end-tier branches sometimes, and esp. with huge hulls, alot of speed and a good ATT/DEF-ratio the DL's will be history. They usually leave ony 2 or 3 Defenders in orbit and stuff their fleets near their planet. And you can lure these ships away by simply bulding a base somewhere in the corner of the map, and once these ships are gone simply destroy the Defenders and invade their world.
