The League vs the Star Empire of Manticore: Would Manticore's strategy be defensive or offensive?
If war breaks out, will Manticore pursue a basically defensive strategy or a strategy of all-out offensive action?
Sort of by definition, any Manticoran strategic stance is going to be "defensive" in the sense that Manticore doesn't want to go to war with the Solarian League at all. Manticore has absolutely no interest in conquering or attempting to rule something the size of the Solarian League, and the Star Empire would vastly prefer to avoid any hostilities with the largest customer for its services.
I realize that that wasn't actually what the question was getting at, but I think it goes to the heart of how Manticore is likely to answer that question. Of course, the way Manticore answers it will also depend on how Manticore perceives the relative combat capabilities of the two sides, as well.
First of all, Manticore is going to approach the possibility of war with the Solarian League with two overriding thoughts in mind. Well, possibly three, since the first of the two I had in mind split a little bit into separate considerations.
The first Manticoran consideration is going to be the need to avoid any possible confrontations with the Solarian League precisely because the last thing Manticore wants is a war against the League. Please note, however, that the real operative word of the above sentence is "possible." That is, Queen Elizabeth would probably phrase her orders as "Admiral, avoid any possible confrontation with the Solarian League, but remember your overriding responsibility to safeguard the lives and property of Manticoran citizens. If it is impossible to avoid a confrontation in the course of meeting that responsibility, then I expect you to confront the Sollies rather than backing down." So, while Manticore is eager to avoid a war with the Solarian League, it isn't sufficiently desperate to avoid that war to compromise its national sovereignty. That's the second point which splits off from the first one. Or, perhaps, completes the first one.
The second major point, however, is that Manticore is well aware of the fact that the Solarian League has traditionally been essentially a powerless giant where its foreign policy is concerned. (This, frankly, is one reason why Queen Elizabeth is so willing to court a " confrontation" with the League rather than compromise Manticoran sovereignty. Mind you, she'd do it anyway, but the extreme handicaps the League faces when it comes to crafting any sort of coherent foreign policy is definitely a part of her thinking, as well.) The fact that any full member of the League has the constitutional power to veto a declaration of war is a part of Elizabeth's individual and the Star Empire as a whole's calculus where the League is concerned (and those of you who have been thinking that the Beowulf connection is a part of that might just have a point. Tum-Te-Tum-Te-Tum.). It's extreme unlikely that the League would ever get around to formally declaring war on the Star Empire of Manticore. It is far more likely that any "war" with the Star Empire would be prosecuted on a sort of ad hoc basis. As an "undeclared war" or a "police action" -- quite possibly a technically illegal police action, fought by the various bureaucracies without any authorization from the elected government at all. This sort of bureaucratic overreach has occurred before in the history of the League, and, in fact, with increasing frequency over the last century or two.
An "undeclared war" with the League could, in many respects, be the worst possible scenario for the Star Kingdom. If the League formally declares war, then the Star Kingdom would be freed to act as a belligerent against another belligerent, which would include attacks on Battle Fleet formations that were still in mothballs, raids on planetary and orbital infrastructures far from the original area of confrontation, commerce raiding against the Solarian League's merchant marine, etc. But if there is no legal declaration of war, and if Manticore proceeds as if there had been one, then the possibility that something like a full-scale declaration will come into existence is far more likely. In addition, if Manticore starts raiding targets far behind the front, light-centuries away from Manticoran territory, in the absence of any declaration of war by the League upon the Star Empire, it will become far easier for the Star Empire's opponents to "spin" Solarian public opinion into viewing the Star Empire as the aggressor. In fact, Manticoran strategic offensives against the sources of the League's military muscle could be portrayed as a deliberate escalation of what had been only "an incident" up to that time. They could also be used to convince the core worlds of the Old League that they and their citizens were actively at risk, and that the Star Empire had become an "overreacting interstellar mad dog" which needed to be put down. What this means is that the all-out offensive stance against the League which some people are recommending holds the potential to turn what might be a smaller scale, RMN-versus-Frontier Fleet confrontation into the sort of all out war Manticore badly wants to avoid.
By the same token, it is going to be quite obvious to the Star Empire' strategists, especially once they have an opportunity to fully evaluate the qualitative differences between the two sides on the basis of actual combat reports, that Manticore currently holds a quite probably decisive edge in war fighting capability and that to stand upon the defensive for any period of time will give the Solarian League the opportunity to recognize its own areas of inferiority and start rectifying them. A Frontier Fleet which the RMN can dominate, hold in check, or even destroy because of Manticore's technical advantages would become far more threatening once those technical advantages begin disappearing, so even a so called "limited war" or "police action" could very well, in the fullness of time, produce fatal consequences for the Star Empire if it goes up against the Solarian League.
There are, of course, still other considerations. For example, a "limited war" which stretched out over a period of, say, 15 years, might well give the Star Empire time to significantly build up the industrial capacity of the Talbott Quadrant and the new territories in Silesia. It could also give the Star Empire time to settle its differences with the Republic of Haven, or even possibly draw the Republic into a mutual defensive alliance against the possibility of Frontier Security moving into the Haven Sector. And, there's always the question of what happens to Manticore's carrying trade and the wormhole junctions. If Manticore is involved in any "police action" against the Office of Frontier Security and Frontier Fleet in a single sector of the Verge, then it is highly probable that the rest of the League (or, at least, a majority of it) would remain open to Manticoran commerce, and the fact that Solarian cargoes were being carried in Manticoran hulls would probably tend to mitigate against a concerted policy of commerce raiding directed against Manticoran merchant shipping.
All of these factors (and probably a bunch more I haven't even touched on) have to be considered by the Manticoran policymakers, both civilian and military. As I hope you can see, the fact that they do means that the arguments in favor of a defensive or an offensive strategy are not quite so clearcut as many of their proponents appear to think they are. And they are also the reason that even if I wanted to (which I don't, frankly) I really couldn't give you a definitive answer at this time on what sort of strategy Manticore will/might pursue in the event of a war with the Solarian League.