Dealing with hostile alliances

[GC13]

Is the possibility for a peaceful resolution to the game meant to be over if you wait more than... What is it, a year or two after getting a third Federation member? Either way, after that your hostile alliance should form quickly and automatically. According to this comment by Keith seems to say that you cannot pry a species out of their alliance, as they won't join the Federation once they've joined the non-Federation alliance.

So once that alliance forms, are you reduced to destroying that species?

[GC13]

Come on, nobody knows if there's a way to deal with these guys short of killing 'em all?

[Billick]

What I saw in my game (and maybe these are bugs):
1) An opposing alliance (the UIS) formed almost immediately after I formed (less than a month)
2) I was able to pull the Burlast over to the Federation by pressuring them.

[topper]

Come on, nobody knows if there's a way to deal with these guys short of killing 'em all?

In previous versions, it was possible (at a high cost) to get other alliance members to join the federation. The comment you posted is for the most recent release, so the devs will probably have to clarify unless someone is at a convenient stage in their game to try it.