A joining ACS fleet has to be synchronized with the battle time established for the group. This is what lets the joint attack resolve as one battle context rather than as several independent attacks.
How does this rule work?
When a fleet joins, Inter-War compares its planned arrival with the group timing. Correct alliance membership and an open slot are not enough if the fleet cannot be aligned with the required time.
What exactly does the server validate?
Travel time depends on distance, the slowest ship, drive technologies, selected speed percentage and active bonuses. Fleet composition can therefore determine whether a fleet can join an existing ACS in time.
How does this affect the mission?
For larger operations, set the intended battle second first and then choose participant speed and composition. Launching without a timing plan can make a later join impossible.
When does it matter?
ACS does not create a separate, looser combat system. A joint attack still uses normal player protection, attack limits, target validation, fuel and fleet-slot rules. The group only synchronizes several valid fleets into one battle context.
Common mistake
Before joining, verify the leader, target, object type and planned battle time. Most problems come from trying to join after the target changed, from outside the required alliance, or with a fleet that cannot arrive at the required time.
Related FAQ: Who can join an ACS attack? · How many slots does an ACS group have?