The Alliance Combat System (ACS) permits fighting together with other players, so that no player is ever on his own. In one situation, you can join your friends in taking out larger targets, or you can defend a friend to intercept an attacking fleet. Both of these situations provides a huge advantage offensively and defensively.
How do you attack with someone else?
To start an ACS-attack, one of the attackers must start an attack mission against the target. As soon as it is launched, he can change that fleet to a union and invite other players via the fleet menu, which allows you to invite others even if the player isn't in the same alliance or on the buddy list. Those players will get an invitation to join union and will be able to join the attack with their fleets via clicking on the drop-down menu on the second fleet page ("Combat forces"). In total, five players can join the union; and the union can consist of a total number of 16 fleets. But look out; all fleets of a union are flown with the slowest one. So, if a slower fleet is added to a union, the arrival time of all fleets will be set back. That can also be used to your advantage, which is explained in Slowdown by ACS.
If a fleet is more than 30% slower than the flight time of the whole union, it will not be able to join. First off if the purpose of an ACS is to intercept a fleet, it is important that the fleet union arrives at the right time. The ACS-attack should be arranged accordingly. It is best if the player who is the furthest from the target starts it. All the others can send their fleets, without reducing speed of the first fleet. At interceptions you also can save deuterium by starting/timing the defend with a single ship at first, and after that joining the combat fleet. By doing this, if you have timed the attack wrong, you will have to call back only one ship and not recall the whole fleet and relaunch again. You especially will have to pay attention, if the attack is cancelled. In that case, every player has to call back his own fleet manually – if only one fleet cancels the flight, the rest of the union will just continue. If the fleet, which starts the union, is called back, no other fleets will be able to join the union.
With a union you can surprise an enemy not only with an attack, but also with a defense. If you want to defend a planet of a member of the alliance or of a buddy, you just will have to command your fleet there and to choose the mission "hold". Before sending your fleet, you will be able to specify how long it shall stay at the target. As long as it is deployed there, it will participate in every battle against any attack launched at the defender. But, the owner cannot control the defending fleet himself. But while defending, the fleet constantly needs deuterium, that is why the holding time is limited by the cargo bay of the fleet. If you want to fly back earlier then planned, you will only have to click on call back in the fleet menu, to cancel the holding operation. A maximum four players are able to defend foreign planet/moon; with a total of 15 foreign fleets allowed.
But look out; if you attack a planet or a moon, on which one of your fleets is defending, your fleets will fight against each other.
The ACS defend is perfect when a member of the alliance is attacked by another player. Here, with a little bit of luck, you can deploy your fleet on the target before the attacker arrives and thus surprise him. If the own fleet arrives a few seconds before the attacker’s one, he suddenly will be face to face with an unforeseen superiority and lose his own fleet, instead of demolishing the fleet of your ally.
What does the alliance depot do?
The alliance depot acts as a deuterium supply for a defending fleet while it is defending a planet or a moon, so that it can stay there longer. To do this, select the alliance depot on the planet which is being defended, and select the amount of deuterium you want to send, which shall be sent to the fleet. The bigger the depot, the more deuterium you can send per hour like that. The alliance depot is considered useless, because you normally do not hold your fleet on a foreign planet for more than one hour. So, you can save time, resources, and a building slot on your planet for more important uses