Relations between legions can be thought of as a few different flags or states. Based on some of the issues I've seen, here are the states that can be formalized:

War - You actively seek out pvp against this legion (say, they appear on your list more often or something when eligable), take their planets, or attack their base (when that's an option). You wouldn't trade with this legion or help it in any way. Their planets are red on your list or something (if that feature existed).

Neutral - You pvp normally and invade when you find something good, but don't go out of your way to attack them.

No-invade pact - same as above except you agree not to invade each other (and are warned before doing so). you pvp normally (something that's been requested a lot as a formal option). Enemy planets of theirs are green or blue or something.

Non aggression pact - no invade + warning about pvp, as before. Enemy planets of theirs are green or blue or something.

Trade pact - NAP + allows legions to share items, goods, and minerals in a third list (so you have legion, friends, and new "trade" list of players to send something to in the trade section). Also opens the base theoretically up for trading cargo you've listed for public trade (if/when that becomes a commerce option).

Alliance - theoretically the next level. Perhaps they can guard your planets or something. Appropriate for "sister legions" which most larger legions have. Something legions can do to support each other, perhaps moving between more easily, or better to avoid moving some way to cross alert something. Not sure how that would work.

+1