It was awful. He kept complaining his planet was being attacked by 8 advent bombers and asking me to do something. Problem: I was in another solar system. He had a massive fleet ONE JUMP AWAY. He lost and recolonized that planet about 5 times. I was ready to shoot myself. Completely useless, didn't do anything worth having him for all game.

On the plus side, all he ever wanted me to do was attack the guy I was already attacking. That's why we allied in the first place.

Once you have the ally, use it as long as it last - as in: Find a target faction that your ally likes to attack and keep sending it. I've used my allies to destroy other factions, you just need to micro-manage it. If there isn't one then wait & try again later. The old alliances might be in effect. Do the missions to get one but you need a little luck with them, 85% is enough so you can command it around.

If your ally is a good tec faction (aggressive attacker) the enemy planets are doomed under siege frigate raids.

Of course, playing with diplomacy is your friend imo...
i mean I'm currently playing a game as TEC with 6 hard AI unlocked teams

and all i keep doing is completing missions for all of em, result is having em all in my pocket, then when i see one of em is becoming too strong, war time... unleash all my cannons unto his planets a solar system away but still... and order all my allies to target him by selecting his ships and giving him rally points, and THEN i come with my fleet, which doesn't hold much power

as a result, i have only allies and i slowly kill em one by one by declaring war on one of em..
as one falls i colonize everything giving me more economy more artifacts ( still want to achieve the goal to have all artifacts at once ) and of course, more Cannons to auto cast lol ( currently got a total of 22 cannons on autocast, yeh ok, it can take nearly half an hour before all missiles stop reaching a given planet so i can colonize it but... shit, good way to slowly exterminate em all!)

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Oh and regarding your question just above, depending on the AI and what he requires, he will give you different missions such as
-Give me X Crystal
-Give me X Metal
-Give me X Credits
-Destroy X ship of the player Y
-Destroy X defensive structure of the player Y
-Destroy X logistic structure of the player Y
Those are the ones i saw... you mostly see em ask for ressources for early reputation gain from my past experiences... and after like 60% or so, all they ask you to do is destroy opponent factions so... but thats from what i noticed.

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And in order to keep an ally... simple, do the missions he requires you to do... oh and by the way, when they ask you for ships to destroy, i noticed that you can kill anything from that player's color and it will count as a vessel... doesn't work for the other 2 missions though, unfortunatly