Well, the title explains it but I will try to describe the situation:

I am allied to the Unfallen (AI) and we are at war against Hissho.  I managed to build an Obliterator and fire at the Hissho (AI) home system.  Due to the size of the galaxy it would take about 10 turns to register a hit.  About halfway down the road I realized that the idiotic ally of mine sent a rather large fleet to siege the Hissho home system.  In about five turns that large allied fleet will bite the dust along with the Hissho home.

Is there a way to tell my ally that he should hightail it outta there?  You know, save the trees and all that :)

Thanks for your help folks.

Unfortunately there's no way to alert your AI ally to the impending doom of their fleet (at least as far as I know). You might be able to "trick" them into leaving if you impose a truce upon the Hissho so your ally has no reason to continue sieging the planet and would leave the system but then you have to wait several turns before you can wage war again and this is expensive in terms of influence. 

I would just let your ally's ships get obliterated, honestly. 

More's the pity.  Thanks for your input.

I could imagine to share view with AI (when Obliterator is discovered before launching rocket) could lead to what you aim for.

Isn't it shared for allies automatically?

Trouble is, everything happened in a rather unexpected fashion. As I said I was allied with the Unfallen (thus sharing vision) long before I fired the Obl. The enemy system was also bereft of any ships, friend or foe. So I decided to go for it. The launch was even publicized much to the distaste of other AI empires. Not that I give a rat's posterior of course :)

Anyway, two turns passed and to my horror I saw my ally's fleet heading for the enemy system. For the next two turns I watched helplessly while they peacefully cruised down the starlane to reach their destination and start the siege.  

Right now the Obl missile is 5 turns away and I am seemingly out of options.  I imagine the Unfallen will be quite upset should their primary invasion fleet meet its demise due to collateral damage from friendly fire. I am thinking about following Valadeus' idea of forcing truce...  Let's see how it'll play out 

Valadeus wrote:

Unfortunately there's no way to alert your AI ally 

Sublustris wrote:

Isn't it shared for allies automatically?

Missed the ally part. Thought about not allied AIs to share with. Thanks for pointing out.

I always felt that oblits need a more obvious cue as to where the big death bolts are heading, and from where. We need a big ominous sound cue.

Aside from that, you can blow your hapless ally kisses. Friends are expendable, or so I play....

TheFunMachine wrote:

I always felt that oblits need a more obvious cue as to where the big death bolts are heading, and from where. We need a big ominous sound cue.

Aside from that, you can blow your hapless ally kisses. Friends are expendable, or so I play....

Yeah...  I have tried the truce option but Hissho nation was a tad stubborn.  They blabbed something about honor and declined.  Eventually my Obliterator missile reached its target.  The Unfallen allied fleet was blown to bits along with the home system of our common enemy.  An unsuspecting colony ship of another empire passing by got caught in the blast as well.  My ally, despite his whole fleet been annihilated, did not even bat an eye.  The owner of the little colony ship did not make a peep either.  One turn later Hissho guys appeared at my doorstep with a truce proposal.  Whether this was a realistic outcome or not, I ended up coming out on top.

I think the diplomacy part needs a little fine tuning in this regard.

Bah, I wouldn't worry about it.  The AI isn't your ally.  It's just doing whatever it thinks will make you lose, "alliance" or not.  I've had more plans screwed over by the AHEM "incompetence" (because let's be honest, it did it on purpose) of my AI alliance trucing at the wrong instant, surrendering a war we're winning and forcing me to pay ungoldy tribute to an enemy I'm PERSONALLY stomping into the dirt, picking the wrong side in the lodestone quests (seriously, 3 AI allies ALL waited for me to pick then picked the opposing team, wtf) or declaring war on an enemy right across the galaxy from them but right next to me, etc etc.

I've said it before and I'll say it again:  the AI is NOT your friend.  It's just trying to stop you in a less overt manner.  You ally with an AI to ensure it'll leave you the hell alone while you stomp someone else, then drop it once it's convenient.  Otherwise you're just giving the computer an inside line to your plans and another way to completely derail you.  If it's stupid enough to send a fleet to your obliterator target then it doesn't deserve your pity; it was likely trying to sacrifice that fleet so it had an excuse to break alliance or go right to war, Unfallen or not.

Screw 'em.