What if, as a result of the Trent affair, UK decides to intervene. But what if their intervention is limited, not wanting to get into a messy ground war, they just commit the Royal Navy and effectively lift the Union blockade for the CSA.
If the CSA didn't have to worry about a blockade and was able to trade freely with Europe, is this enough to change the ultimate outcome?
Well its likely to greatly length the war, at least
unless Lincoln comes to terms quickly. I'm assuming their lifting the blockade of the south as a bi-product of blockading the north as otherwise their leaving themselves open to union commerce raiders. In that case.
1) Down factors for the union, in no particular order:
a) Loss of a good bit of their navy, even more if it tries to challenge the blockade.
b) Loss of any pockets/enclaves in the south as their no longer supported.
c) Loss of overseas trade and also revenue from tariffs, which was the largest source of government revenue in peacetime. Doubt it will be a total blockade given the standards of the time and also Britain will find it useful to import grain from the US.
d) A serious run on the dollar due to even a limited war with the UK and what foreign loans are available being an a markedly higher rate.
e) Loss of access to European guns, powder and other supplies useful for the war.
f) Probably a lot of volunteers from Britain and Canada won't be coming and at least some of those in the union already will seek to return home. Also goods from Canada including a lot of horses are unlikely to be available.
g) Expect at least some union attacks on Canada as that's the only way the union can try and respond, other than calling uncle immediately. Even if they don't some forces will be sent north in case of a possible British attack and you might get some forts and warships built for possible conflict there. This all means men, guns and other resources will be pulled away from the war in the south.
h) Probably the loss of a lot of their merchant fleet and fishing fleet as its either reflagged to avoid the blockade or laid up. Insurance costs are likely to be a factor here as well.
i) Likely to see a significant drop in immigration to the union during the duration, which will reduce manpower for the military and general economy. This could be countered to a degree by people involved in maritime activities, many of whom will be losing their jobs if the blockade is prolonged.
All in all a significant drop in military potential and a serious economic and fiscal crisis, probably resulting in pressure to come to terms with Britain and/or the south. There is the possibility, especially if propaganda gets in the way that the union population could get very hawkish, in which case there is a more general war - as ultimately its not up to the UK on its own to decide their only going to have a naval war. This is likely to go badly for the union however unless the UK really, really fouls things up.
2) Pluses for the south
a) They can trade internationally, selling cotton, once the initial international surplus is gone, and more importantly getting the guns and other military equipment they have problems with OTL.
b) It also means the south can use coastal shipping to ease the pressure on their weaker railway structure, and also probably get more food from fishing activities.
c) Once the assorted enclaves are mopped up now their isolated the south can in theory send a lot more men, which are likely to be better equipped, to fight on the main northern fronts. Divisions between the states and the need to have some precautions if Britain and the union come to terms, exposing their coasts to attack again would require them to keep some reserves in the south but for a while at least their likely to have more manpower. Whether they use that effectively is of course another factor.
d) The lifting of the blockade and even a limited UK/union war is likely to boost both southern morale and the expectation it will win independence in the wider world, making it easier for it to get loans I suspect.
e) Depending on the timing you might not see Monitor being moved south from New York so how long before the Virginia/Merrimack can be opposed and what impact that might have?
f) This is
only a possibility but given the UK/union tension someone, probably France, might decide to recognise the south which opens a serious problem for the union. IIRC Lincoln's government had the stance that such an action would be a casus bello but could they afford to do that in such a circumstance.
Overall the direct pluses for the south are probably less than the negatives for the north and a lot would also depend on how long the state of war with the UK lasts. Even say a couple of months, as well as a fiscal and economic shock for the union would allow the south to get a lot of imports in and if the union lost all its bases in the south resuming the union blockade would be markedly more difficult. A longer blockade, say six months
or more would give the south a much stronger position that,
unless they manage to waste this, will at the very least make the war markedly longer and costlier for the union before it can decisively crush the south. The obvious danger here would be if the situation is markedly worse for the union Lincoln loses the 64 election to a peace Democrat. If it became a full shooting war with the UK the conflict is unlikely to last that long and the union will suffer a clear defeat.
Anyway initial thoughts on the issue.