I see the Andurien attacks on the Confederation as an emotional act, not one driven by logical action. It's all tied to the mess that is the Daoshen/Ilsa/Ari triangle.
The meeting between Daoshen and Ilsa, in my opinion, went far a standard meeting between two allies, emotionally speaking. In that meeting, we saw chinks in the armor that both always had in place. Daoshen shows jealousy over Ilsa's frequent machinations with Ari, even though the Chancellor was the very person who suggested the marriage in first place. Ilsa, on her end, believes that she had always acted with the Confederation's best interests in mind, and is deeply betrayed by Daoshen tossing her aside because of said unfounded jealousy.
Ari, however, is a man that Ilsa still has complete sway over. A pony express is set up, and troops that had previously been preparing to assault the Free Worlds hit the Confederation instead: an act of reactive passion, performed by a Duke who was probably never aware of the truth behind the switch in targets. Put clearly, a hurt Ilsa used the Duke to strike at the one thing she knows Daoshen still holds dear—the Confederation itself.
Now what? In a certain parallel to 3025, Andurien has become 3150's best example of the Piranha Principle at play. While both the CapCon and the FWL would love nothing more but to absorb the nation and remove Ari from the throne, neither can spare the forces without facing the risk of tremendous losses on other fronts. Andurien doesn't have a reason to join a new Star League because of this, unless one or both of its larger neighbours do first—freeing up those essential troops. Interestingly, if Andurien feels like one of its larger neighbours might join the League sooner than later, it might join first to preempt any mass invasions; ultimately speaking, its next steps could be guided by pure diplomacy more than raw military action.
