The key point is that such a powerful civilization within the Thousand Alliance quietly left right before their eyes, which inevitably damages the interests of the whole Alliance.
It's important to understand that even if internal conflicts are intense, the interests only shift within the Alliance. Once the other members become aware of the situation, they have no choice but to accept this reality. However, if the entire Xisoke Civilization is forced to completely side with an unknown civilization, causing the disappearance of all intelligent civilizations and valuable resources in this area, it also gives the other strong civilizations within the Alliance a reason to attack and punish them.
Moreover, they're unsure if this mysterious force, after gaining the Xisoke Civilization, might turn its focus towards them.
No matter what, the situation has evolved to a point where they can't ignore it.
