The League vs the Star Empire of Manticore: What would happen to the Manticoran merchant trade?
In the event of a war between the Star Empire of Manticore and the Solarian League, what happens to the Manticoran carrying trade and the wealth generated by its wormhole junction?
This is another question which admits of several possible outcomes. First, bear in mind that the Solarian League's shipping lines also rely upon the wormholes dominated by the Star Empire's astrographic position. Also bear in mind that the tactical superiority of the Manticoran Navy, and its ability to move quickly via the wormhole junctions in the event of hostilities, means that Manticore would be substantially better placed militarily to secure (and retain) control of the majority of the wormholes in existence than the Solarian League would be.
Another not unimportant point to bear in mind is that Manticore has gone to considerable lengths to build and maintain the friendliest possible relations with the other wormhole junctions. It hasn't always been possible for them to nourish good relations, generally because of other circumstances, but overall, Manticore is on excellent terms with the majority of the star systems which possess wormhole junctions of their own. Many of these systems are members of -- or, at least, dependencies of -- the Solarian League, which naturally means that the official policy of the League is going to become a factor in their relations with Manticore. Despite that, however, it's most unlikely that any of those star systems would be willing to cooperate in shutting down Manticoran traffic through them except in the wake of a formal, legal declaration of war from the League Assembly, rather than simply some sort of bureaucratic fiat handed down by Frontier Security or Frontier Fleet. And, as I already commented above, in the absence of such a formal, general declaration of war, it is entirely possible (even probable) that Manticoran trade with the majority of the Solarian League would continue unabated.
(As a historical example of how this could happen, look at the relationship between the Dutch and Spain during the lengthy rebellion against Spanish rule in Holland. Spain was one of the Dutch merchant fleet's main customers. In fact, many of the artillery pieces mounted by the vessels of the Spanish Armada had been cast in Dutch foundries. Business was business, and the governments in question winked at it because each of them needed something the other offered. This particular relationship existed despite the fact that Spain had declared the Spanish Netherlands to be in a formal state of rebellion and had dispatched an occupying army to fight a particularly brutal war on the Netherlands' soil. The possibility of a commercial relationship continuing untrammeled by the vicissitudes of a "state of war" which didn't even directly impinge upon the star systems in question would, I think, be considerably greater.)
Assuming that a formal state of war was declared, and that the Solarian League remained intact as a coherent political unit (or, at least, as close to a "coherent political unit" as it's ever been), then the consequences for the Manticoran merchant service -- and for the revenue stream based upon that service -- could become far graver. Even then, however, the fact that Manticore would be in a better position to control/dominate the network of wormhole junctions would mean that cutting off Manticoran trade might well prove to be a case of having cut off its nose to spite its face where the League was concerned. Should the League succeed in cutting off all Solarian trade with Manticore (which, frankly, would be extremely unlikely, in my opinion, given human nature), Manticore would be well positioned to inflict severe, if not outright crippling, damage on Solarian commerce simply by denying League shippers access to the majority of the wormhole network. Shipping times would increase (you should pardon the expression) astronomically at the very same time that a huge chunk of the hulls (the entire Manticoran merchant marine) -- not to mention the warehousing facilities and service organizations -- upon which League shippers have depended literally for generations evaporated. The financial dislocations within the League would be immense. Although probably not life-threatening to the League, on the face of things, at least, it would inflict severe damage on the League's industrial and economic stance, it would throw the entire League badly off-balance strategically, if only where the movement of military units were concerned, and the severe economic hardship it would impose upon the systems most affected by it would be a powerful be stabilizing influence on the League because of the way in which it would undermine those systems' loyalty to and self-interest in the concept of preserving the League.
The consequences for Manticore, would obviously be extraordinarily drastic, as well, of course. The question which would arise would be the extent to which Manticore's presumed increase in trade with the Republic of Haven and the Andermani Empire, plus whatever sectors/segments of the Solarian League wound up effectively isolated from the rest of the League by Manticore's domination of the wormhole networks, compensated for the loss in trade with the rest of the League. I'm not beginning to suggest here that those other sources would equal the loss in trade with the rest of the League, although I strongly suspect that it would increase drastically, since Manticore's continued existence would imply that it was managing to lop off additional chunks of the League and incorporate them into at least its economic sphere of interest, if not into its territory. I'm simply saying that there would be some compensation in the new trade realities, and that the hit to the Manticoran revenue stream might not be quite so severe and crippling as might be expected at first glance.
Frankly, it's also highly likely that Solarian shipping would suffer more from commerce raiding and privateering than Manticoran shipping would. There are several reasons for this, including the fact that the Manticorans have far more experience at commerce protection than the Solarians do and, in ships like the Rolands, they have much more effective commerce-raiders. But the biggest single factor would probably be Manticore's likely domination of the wormhole networks. Essentially, Manticoran merchant shipping would be far less exposed than anyone else's because it could go directly from one star system to another, hundreds of light-years away, in a single jump, whereas the Sollies would lose that capability. The greatest areas of vulnerability would still lie within the star systems of destination for the cargoes in question, given how difficult it is to locate a convoy or a single freighter in hyper-space, but that would be a point at which Manticore's greater experience and commerce protection would come into play most strongly.
I suppose the bottom line is that in the event of a complete rupture between the Solarian League and the Star Empire of Manticore, I think Manticore would probably be in a position to increase its domination of the carrying trade and economic services market by expanding into the areas the Solarian League was no longer capable of servicing, since there would no longer be any Solarian competition in those areas. Obviously, this presupposes that the Star Empire is able to survive militarily and to exert control over the wormhole networks as I have suggested above.