pendragonman:
There are very few polities dangerous enough to prevent a 3I charge in and smash the pirates: The Zhodani, the Solomani, and the K'Kree come to mind.
The Julian protectorate could survive a local push; but in repelling it, they're likely to wind up stirring up the 3I, and get eaten by the combined mass of the domain.
The Sword Worlds and the Darrians can't; the Swordies, canonically, had lots of help staying extra-imperial. The Darrian tech edge over the imperium is a fallacy (propaganda, to be specific) and almost meaningless in CT or MT terms, and pretty trivial in TNE terms.
The Solomani are unwilling to provoke further Imperial agression until they have built up enough to retake their "Home-world" from the Imperials, and further, they were imperials (in an autonomous region) for much of the 3I.
The K'Kree know they can't handle the 3I as is; they need some other BIG conflagration to be able to achieve their goals.
The Zhodani keep TRIGGERING frontier wars... to keep the local imperial fleet from thinking they can just whomp upon the Zhodani. These wars are expensive enough that Capital notices.
The Vargr Extents, however, present a pricklier problem: any one vargr state could easily be toppled. But, the more one topples, the more the others will gang up to get rid of the überhund. A unfied vargr extents is not something the Domain can handle, and the extents can easily cut the domain of deneb off. So toppling the locals isn't doing. But, by the same token, not a one can stop you from pursuit if you intend to, but they can and will make it painful, and be generally uncoöperative in the process.