I’m saying reconstruction wasn’t successful at changing the culture or improving things in the region like they said it would. From a political and economic standpoint yes it was success. From a moral and ideology standpoint it was a failure. The north benefited greatly from it. Southerners and blacks not so much. Much the the region stayed second world like until the 40s. Appalachia was like the second world until very recently. The south is kind of has a banana republic type relationship with the north until the 40s to 60s. Also my point about troops is not their willingness to fight during a actual war but prepare during peacetime. People in New England and Mid-Athletic would not want to have conscription imposed on them. They would riot if the union tried to make 2 to 5 years of military or civic duty mandatory. The south would do that. They will not invest that much time into that. Yankees will not want a good portion of their young lives to be stuck doing military service. The south might impose mandatory service for 2 to 5. People who do more years get more benefits and pay. Many poor whites could voluntarily stay in the military for up to 8 years. They will have high voluntary numbers because many people would see it as the only way of moving up in the world. Some still do this. People in the south will be more willing to serve for much longer periods of time. They probably get a lot of action too during peacetime acting as a national guard and police force too. Who do you think they will need to put down possible slave revolts, partisans and bandits in Appalachia, Native Americans and outlaws out west, and that only increases with expansion. Blacks might be used as a excuse to keep a massive standing military/police force even after slavery ends. Their society would become more Spartan like. That is a lot of experience. They will be more mobilized and prepared to start off with. That’s kind of bad if the US has to mobilize while the CSA basically already are and are even adding more. The CSA technology might not be that good but a lot of major cities aren’t far from the border. It’s hard to mobilize if some of your major cities get captured right away. Do you think Yankees are willing to commit that much to the military? If let’s say 10 percent of white southerners are a full time paramilitary type force while under 1 percent of the north population is currently military ready at the start of the war how does that close the gap at the start? They will build up a large navy and a very well equipped and trained army but the CSA could have a much larger portion of their population committed to military life while the voluntary/draft based north has a much lower percentage on that. That might lessen number gaps in the early war between the two. The north would have to make sure it’s standing army isn’t overwhelmed by confederates early in the war. If they do that they win. If their lines break the Confederates could take DC and maybe Philadelphia. West Virginia could hold. The mountains make it were numbers are less of a factor. Kentucky gets overwhelmed but the River actually stops any further advancement(Kentucky south state line isn’t defendable at all especially when technology improves). Maybe parts of Missouri and plains too. The confederates have to depend on winning early every time. Maryland and Delaware would be pushing it but given the chance I could see them doing it. They have to overwhelm and blitz as much as possible early on to win. This just needs to be successful once to improve CSA chances in the long run. The north might think they did all in their power to prepare but they didn’t think they needed to go to the extremes the CSA did. This isn’t a liberal republic. This is Putin Russia in the 1800s at best and proto-fascism state at worse. If they take Kentucky and West in the 1800 century that is more defensible borders and coal later on. CSA might be backwards but they could still have more Calvary ready to start a war. Even in the 1800 century Calvary units could likely go through the Great Plains pretty quickly and capture rails and cutting the US in half. Geography might hurt them here if the CSA is a militarized state with men ready on all fronts to some degree while the US has a lot of land that would be impossible to guard without investing a lot of manpower and resources. A lot of borders are literally straight lines between the two. Slavery does free up and likely encourage many whites to join the military. The north just had to hold out until their reserves come in to reinforce them. The war could start out with the confederates out numbering them 12 to 1 or more but the war could end with the union outnumbering them 20 to 1 if not more. But even with outdated weapons they are probably still very efficient soldiers given the system described. If they are successful at expansion they are going to become the Russia of the Western Hemisphere. They will be seen as this aggressive nation but backwards nation who many dislike but some work with. They depend heavily on raw resources which given the amount they have they could get some useful friends. I do think a divided America might be seen as weaken or a easier target by European powers. They might be getting more involved in the Americas with the US being more divided. This is still the age of imperialism. They probably won’t be trying to take over stuff directly like the rest of the world but they might try to create allies, business, and even puppets more so then otl. I could see Germany and Britain playing people within the Western Hemisphere in this situation.
Edit: It is also important to consider states like this can more easily survive in this era especially within the Americas(abundance of natural resources. Market demand will win some allies). International cooperation of post-ww2 isn’t here yet. States like this would be slowly isolated and pressured into collapse in most situations post-ww2. Any regime can survive a long time when they always have a good number of nations to trade with. They might use their raw resources to get enough weapons stockpiled and ready especially with Europeans willing to feed into conflict. The CSA still have a lot of valuable raw resources.