I'm about to start work on a sci-fi tank game. I want it to be realistic as far as most physical aspects, and some technological aspects, minus some FTL travel (using something similar to StarGate's system but more limited), FTL communication, and possibly some energy weapons. I expect and plan to be able to have decent multiplayer gameplay. I mention multiplayer because a game that is mostly about multiplayer might not need as much of a backstory. Any thoughts?

I agree, no backstory is necessary.

Any backstory should be minimal, and play into the personality of your game. Many games like this rightly use a gratuitously terse, tongue-in-cheek style of story telling. Basically, instead of usual avoidance of cliche, you harness the blunt force of cliche for the sake of humor veiled as adequate motivation to blow shit up with a tank.