If someone declares war on your ally and you don't declare war on the aggressor, the alliance is auto-broken, but your relations otherwise remain the same (or perhaps one less; hadn't tried that specifically).

You're free to re-initiate the alliance later, so long as you're still Close, which isn't quite as easy as it sounds.

I'm not positive, but you may have to wait the 4 or 8 weeks to talk to them again after the alliance is broken, even if it's done by a war.

Do note that you get a similar prompt if someone attacks the Altarians and you are of their ethical alignment (so long as you are neutral or better with them); but this request has no penalty for not complying.