The League vs the Star Empire of Manticore: What happens if a revanchist Solarian League or successor arises?
What happens if a revanchist Solarian League or Solarian League successor state decides to go after Manticore after achieving technological and numerical parity or superiority?
The definitive answer to this question is -- who knows? Huge amounts would depend on the exact status of the Star Empire at the time this revanchist threat emerged, presumably from some deeply hidden Solarian Bolthole. If one assumes that the Star Empire has continued to expand, to strengthen its internal markets, to develop a friendly commercial relationship with the Republic of Haven (which might also be peacefully expanding once again, if it had a chance to get its economic feet fully under it), to negotiate a reasonably conciliatory peace treaty with the Solarian League or, at least, with the majority of its successor states, and to work at improving its relationship(s) with the League or the majority of its successor states, it would have to be one hell of a revanchist force to make much headway. For reasons I've already listed above, I think it's extraordinarily unlikely that any Solarian Bolthole would be able to produce a credible threat (in terms of both technology and necessary numbers) to the Star Empire in any sort of short-term time frame. Assuming that that's true, then one also has to assume that the Star Empire would not be letting grass grow under it in terms of protecting its own territory and position. The fact that they've already been Boltholed and Thunderbolted once is going to keep both ONI and the Admiralty on its toes and wary where such a potential threat is concerned, as well. So given the fact that I don't see this particular threat emerging within anything less than a decade or two of the Solarian League's defeat, and that even then I find it very difficult to believe that a sufficiently large navy could be built up without Manticore's noticing, I don't really see this as a realistic threat.
Now, if the Solarian League managed to establish a Bolthole, or a series of Boltholes, while it was still an intact political unit and to sustain operations against Manticore while that Bolthole did a Shannon Foraker and came up with the new technology the SLN would require to stand up to Manticore, that would obviously be a completely different situation. How that would happen, unless Manticore were to suddenly become sufficiently terminally stupid to allow the League to spin out negotiations long enough, would be difficult for me to envision at this time, but that doesn't necessarily mean it would be impossible. However, that's rather a different question from the one about what would happen if a revenge-minded faction of the League decided to build a navy and come after the Manties.
In addition, I question whether or not any such revenge-minded faction would be a realistic possibility in the first place. First, the Solarian League isn't the sort of government that inspires an intense personal loyalty out of its subjects. If a citizen of the Solarian League has a powerful sense of patriotism, it's normally focused upon his home star system, not on the vast, mostly unseen, totally bureaucratic, and completely politically nonresponsive Rube Goldberg contraption called the "Solarian League." For the vast majority of Solarians, if asked to choose between the interests of their home system and of the League as a whole, and to decide whether his personal loyalty belonged to his home system or to the League as a whole, the decision would be the same as for the US Army officers who resigned their commissions to take service with the Confederacy following their home states' secession from the Union. It's always possible that if the Manticorans were sufficiently maladroit, or if some opposing faction (like, maybe, the Mesan Alignment?) were sufficiently adroit, at manipulating and spinning events, the Manties could end up doing something (or being accused of doing something) which would so infuriate Solarian public opinion that the Manties as an external threat actually brought about the creation of a true sense of League-wide patriotism. Given the current terminal state of the League's relationship with its citizenry, however, that is extremely unlikely. (Which doesn't mean that I couldn't figure out a way to do it if I needed to come up with a fresh credible bad guy to confront Manticore, of course.)