Blue Max
The thing is that the CSA has no easy route to economic development, and its at least a coin flip that the CSA is still a slave society--and in any case, the USA is going to have a stronger economy and yes, its building its army because it is surrounded by enemies. The CSA's cash crop, Cotton, is unlikely to support its economic development, and the UK is probably going to have to station troops in the country.
I see it as highly unlikely the UK would station troops in the CSA, that the CSA would want them short of a new war or that they would be needed. The US is going to have a stronger economy, although weaker than OTL and significantly weaker following the path you suggest. However it was much stronger in 1861 and took 4 bloody years to defeat the south. With that averted you had an internationally recognised, if loathed, state that has set down institutions. It doesn't need to be as strong as the north, just strong enough for the north to have to pay such a butchers bill that public opinion will not favour it.
You seem to think that there will be a universal blood-lust in the US to conquer the south. That won't be the case. Its not even like A-L as the south is not a region torn from the US by a foreign power and held against its will but a nation in its own right that doesn't want to beyond to the US. Many in the US would be unhappy with that but few will want the bloodbath that would result from trying to conquer a neighbouring state.
Don't forget also that we're talking about, as far as I'm aware, a conflict triggered by an enlarged Trent Incident. I.e. the US committed a gross breach of international war then [unlike OTL] failed to correct its error. Many in the US may not see themselves as guilty but the rest of the world will and if they continue to claim the right to rid rough-shod over international law they will leave themselves viewed as untrustworthy and erratic rouges that might be partners in circumstance for a brief while but are unlikely to have close allies.
One other question? If the US did plan to conquer the south, what would it do with the blacks? It was patently clear that many if not most northern whites didn't want them in the US but if it takes over the south its going to have responsibility for them. Not many people may think about this but some will.
A different Civil War butterflying Bismarck? I don't see how. And figure that losing the heavily English south just increased the USA's Cultural Ties with Germany at the expense of its American Ties.
I didn't say butterflying Bismarck, I said butterflying how Germany was united. For instance, a successful Trent War would probably mean the US was so pre-occupied that France continues support for the empire of Mexico. This could affect the Franco-Prussian war in at least 5 ways.
a) A successful campaign gives France extra resources and prestige.
b) Continued problems in the region means that France is too pre-occupied and doesn't take the bait when Bismarck tries provoking it into war.
c) Lessons learnt from the campaign helps turn one of the early battles, say due to new tactics/equipment or a leader who gained experience in Mexico.
d) Conversely one of the less capable of the OTL French generals is not available because he's busy commanding French forces in Mexico.
e) Because their supporting a Hapsburg in Mexico France comes to Austria's aid in 1866. Cue a long an bitter war in which Prussia's bid for domination is defeated and Germany stays divided but largely dominated by Austria.
Not saying any of those would happen but there are so many butterflies that we can't say with any certainty what will happen so far ahead. For instance some have suggested that if Wilhelm II had been born without his handicap he would have been a vastly different character. Might not have been anything like as personally insecure and hence aggressive. Could avoid war or have a totally different set of alliances.
Another point. As you say the US will lose a large proportion of its population, heavily British in background. You seem to think this will make it more attractive to immigrants from elsewhere in Europe, especially Germany. Ignoring the shattered economy, if the conflict is long, the heavy war debts and poorer credit rating and the high military spending and conscription you seem to be suggesting. All of those will reduce the attractiveness of the US to settlers or migrants. Do you know why, when immigration restrictions were imposed after WWI, they used a census from a generation earlier? If so do you see the point I'm making? There might not only be much less of a pull to the US there could be a pretty substantial push away from it by the US government.
So you have a different WW1. It's probably earlier, it probably has a similar outcome to the scenario I suggested, except the arrangement of powers is going to be confused. Germany and the USA are solid--the UK has put its troops in the CSA, and in the eyes of the USA, it screwed the USA and the USA's war efforts are going to be taking out the CSA first. Austria is also solid--Germany and Austria were closely tied by nationalism as well. On the flip side, the CSA, UK and France are all on one side as well.
As I say predicting that far ahead it impossible with any degree of accuracy. Don't forget that Prussia and Austria are allies in 1864 then bitter opponents in 1866. France and Britain spend most of the period in competition and rivalry. Could easily be, if Germany exists in the same shape, that its allied with Britain against the US and France. Which would give huge butterflies.
I suppose this leaves Russia, Ottomans, Italy and Spain without a side. Spain, if the action happens before the loss of its Cuban colony, might be neutral and it might oppose the CSA because of its colonial interests. Italy might opt to sit out the war as well--or it could choose either side. Now, with the UK's designs on places like Egypt, I can't see the Ottomans taking the UK's side in the conflict, although they could be neutral. Finally, Russia itself could be neutral, particularly if the flashpoint isn't Serbia. An American Flashpoint would probably result in Russia trying to gear up for war but remaining neutral anyhow.
Do we know who would take Cuba? Britain like France had commercial and strategic interests in Egypt but if you check the history it was a highly reluctant occupier. However if Russia did stay neutral and Turkey joined the central powers as you suggest then:
a) Britain has relatively little need to attack Turkey, without a desire to aid Russia so could just have small local/colonial forces holding the borders.
b) Austria is going to have to transfer forces to the western front, which with potential crisis in the Balkans and Russia it may well be reluctant to do. [Alternatively it could end up fighting an allied Italy but if the latter is defending, as it probably would be against an Austria not committed on other fronts its not going to be an easy victim.
c) If France is facing Germany largely alone it could well have a far more defensive stance, in which case the Germans could get very, very badly mauled.
As such, even if the clash you suggest developed it would be far from easy for the central powers to get any victory.
So, we have: USA, Germany and Austria vs. UK, France, and CSA. Fundamentally, the USA/Germany and Austria have the stronger hand. So if we want to talk butterflies, a conceivable interpretation is Russia and the Ottomans sitting out a 1905 WWI triggered in North America--Germany Rolls West, crushes France, then focuses its attentions against the UK. Invading the UK probably doesn't happen, but the Germans are having quite a time raiding UK shipping and moving forces into African Colonies. A Distracted Canada and Agarian CSA can't hope to hold on against a built up USA.
A built up US would probably be a military giant, by OTL standards, at the cost of being an economic pigmy. Canada, which could easily have double the OTL population coupled with a defensive position from the CSA would be a hell of a tough pair of nuts for the US to crack. The prairie provinces would almost certainly fall but holding them could tie up a lot of US forces and conquering either BC or the eastern region would be tough even without British support.
The whole Harry Turtledove line is a CSA Wank--which isn't happening: the CSA has been forced to yield considerable territory--such as West Virginia, Kentucky, Half of Tennesse, and all claims to Western Territory. There is no asinine demand for Ohio. The CSA, which either continues Slavery or is in some related practice afterward, doesn't have the population, doesn't have the weapons capacity and it faces internal problems that the USA does not. The CSA is likely to explode in a spate of race-rioting if the USA hits it hard enough, and the presnece of UK Troops--which have to be supplied either across the Atlantic or at heavy expense to the CSA. The CSA doesn't have it, and Germany and Austria will ensure that the UK can't bring it, so the USA owns it.
I would disagree about TL191 being a CSA wank although it's far from realistic. However a question. How long after the US triggers a Trent War does it continue to fight? A short war and the borders would probably be pretty much what they are at the start. However this means little abiding hostility apart from the head-cases. A long war and British annexation of western territory and the US is probably a basket case in the immediate post-war period. In that case the CSA might end up with W Virginia or Kentucky, although I think they would be highly foolish to seek to hold the former as there doesn't seem the support for it.
What could really screw the US is if it does start a 2nd war in the 1880's as in that TL. If so their highly unlikely to get off as lightly as in his books and are likely to be fighting alone.
And for a UK-USA reconcilliation to work, the UK would have to switch its support from the CSA to the USA. This, in turn means that the USA is free to deal with the CSA as it sees fit. In short, it is only with the support of the UK that the CSA can survive--and supporting the CSA means that there will be no acceptance. The USA wants what Canada took from it (although this might be forgivable) and it wants the CSA back. On what grounds can the UK somehow get around these desires?
For a UK-US reconciliation the US would have to persuade Britain it is a trustworthy power. Insisting it has the right to invade and conquer a neighbour would not be the way to do that. That would force Britain to support the continued independence of the CSA as a vital protection for Canada.
Steve