Lit The Treaty of Anaxes

Something I have never understood, but I've always unfortunately pinned it down on me being dumb.

So, the Treaty of Anaxes states that the Fel Empire and the Galactic Alliance shall ally with each other in response to an outside attack, presumably a Yuuzhan Vong-ish threat. How can then the Empire use this treaty to declare war upon the Alliance? I can totally understand starting a war with the GA, for reasons real or imagined, but it seems bizarre to do it by invoking a treaty that refers to the exact opposite situation.

 

By claiming the GA has been taken over by extra-galactic elements? I mean a 'Vichy' GA was even left alive after the war, enduring into Krayt's reign.