Allied Attack

During the space stage you can order an alien empire you are allied with to attack another empire.

How to order an attack of an ally
While contacting your ally on one of their planets there is an option to ask them to attack someone in the diplomacy menu.

After selecting the appropriate item from the diplomacy menu a list of possible target star systems is offered. Upon chosing one of them you are asked to pay an amount of sporebucks. If you refuse the attack is not executed, but there are no other apparent negative consequences.

Effects of an ally attack order
After paying up, your ally immediately attacks one of the inhabited planets of the target star system. The rules for the decision which planet to attack are currently unknown, it might well just be random.

Unless the player interferes after a few minutes all cities on the attacked planet surrender. This result can be endangered just by entering the planet with your ship as difference in the strength of the two empire's ships appear to become crucial. (In general the more star systems an empire has the stronger its ships are with the grox on the top of the foodchain obviously.)

If the attacked planet was the only one occupied in the star system the attacker becomes the owner of the whole star system. If there were other planets with cities on them they are not immediately attacked. There have been a few occurances where an attacked system with multiple occupied planets has switched sides completely. However it is currently unknown how much time had passed between the initial attack on one planet and if the other planets were actually attacked, too, or if the whole system was just handed over without any more hostilities taking place.

Possible Targets
Obviously the list of possible targets only consists of a very few of the thousands of stars in the spore galaxy. The rules of how those are selected are partialy explored and knowing those might prove beneficial to the player.

Range
First of all there is a range limit imposed. It is unknown if it scales with the player's interstellar drive but not unlike it the limit decreases closer to the center. It is currently however unknown (and might resolve the beforementioned uncertainty about scaling) if this persists after visiting the galactic center.

It is known however that systems further away than the range of the best interstellar drive are too far off to be possible targets.

One System per Empire
For every empire that has star systems occupied within range one is selected. Usually, but for unknown reasons not always, the closest one to the planet on which you give the order to attack.

No friendly systems
Systems of the player's empire never seem to be legally attackable, neither are systems of the ally itself.

Only visited systems
Systems which the player has not yet visited cannot be attacked. In order to count as being visited however you do not need to actually enter the system it suffices to fly-by. If on mouse-over on a system you can see the names and stats of its planets consider this prerequisite met.

IMPORTANT: You cannot guide the ally to attack a system by making it the only one visited within range. Apparently the target system for each empire is selected before the check if it has been visited. If it has not been visited the according empire will not appear in the list at all.

Failure conditions and consequences
Below some reasons are described why an order to attack may fail and what this entails.

The system is already attacked
If the selected system is currently under attack by this or another computer controlled empire it is still selectable from the list of possible targets. Instead of being asked to pay you are informed that the system is currently attacked. Nothing else happens though.

A planet in the system has already been conquered by the ally
In this case the system is attackable normally by the ally. However there appears to be a good chance at least that the conquered planet will be attempted to be attacked again, resulting in having lost your investment as no apparent progress comes out of this.

The attacking ally is defeated
This only appears to happen when the player interferes. As described above if left alone the ally invariably wins the attack. However the presence of the player on the attacked planet alone can lead to the allies failure. If that happens the next time it is attempted to order the ally to attack that system the price increases.

Strategy
For maximal efficiency of this function the following strategy is proposed:

Select a system of the ally
This choice is important because it determines which targets will be preselected. If both your ally and your foe are sufficiently large you can order attacks on several systems from several systems.

Scout
Make sure to have visited at least the two or three closest star systems of your target empire. As mentioned above you do not need to enter the system, flying close to it and then changing course might suffice. If you can see the planets of the systems afterwards when you mouse-over on the system in the galaxy map it is enough.

Remove unwanted targets
The most common obstacle to using the ally attack effectively are half-invaded systems left over from previous attack orders. Also you might not want to send your ally to a system you eventually want to colonize yourself or which does not seem a good target out of strategic considerations.

The removal process is straightforward: Fly there and nuke or conquer it. If the system is unoccopied or in your possession another system will be chosen as a possible target instead. If your ally already has conquered one of the planets and you want it to attack another system from a given planet, definitely nuke all the other empire's planets in the system.

As mentioned above just not scouting a system won't help.

Order the attack
That should be straightforward enough if you have read the rest of the article.

If you still can't get the ally to attack the right system remove more unwanted targets or let it attack other targets first. You might also want to consider flying to another planet of your ally and try from there.