Norman said:
I've thrown this out before, but I'll try again. If the US has a different president than Lincoln, and there is a genuine threat to the two nations, such as overly ambitious french plans for Mexico that include annexation of Texas.
And that is something I was pondering, excpet your POD could be much "smaller." A victory at Gettysburg provides the British with the material evidence needed that the CSA has the potential to win, with support. They decide to ally with the CSA.
The French know this through intelligence networks before the CSA or US. Take a look at Napoleon the III, who ruled at this time, and you'll see he was the type of person to try at something ambitious like annexations. I don't think he would go for Texas. Too many people who have too many guns and would get pissed off at having the Mexican Army marching through their back yard. He might try and go for parts of Texas or Arizona or New Mexico. In conjunction he might try and play up the annexations with a nationalist platform to galvanize Mexican politics in support of the installed monarch. If he is successful, a new Mexican "Manifest Destiny" could take root. Always good for a ruler who wants to expand an Empire.
The other consideration is that Napoleon III can not stop the British from supporting the CSA and the potential power shift that might result. But if you can't beat them, join them. The French can play the trump card of being potential allies to the Union. The French have, since the ARW, been on friendly terms with the US and they certainly might accept French help if the British supported the CSA. The British can't stop the French from doing that. The only alternative is an alliance. They help the British gain back lost territory if they help the French-backed Mexican government gain back some. They both help the CSA gain idependence. Maybe Mexico can share in some of the cotton trade afterward.
They decide on a two-sided pincher tactic, launching troops from bases in Mexico and Canada while simultaneously launching blockades against various ports and raiding ships. This goes on for a while with some successes and failures on all sides. The CSA/French/British have made some siginifcant gains in territory, but still have not captured any of the most vital military or industrial centers of the Union. The Union hasn't succeeded in stopping the advance, but has bloodied them quite a bit and minimized the effectiveness of the blockades.
The French, or more specifically Napoleon III, has been engineering a popular revolt in Texas and Louisiana. The Quebec plan was still-born, the Texas one falls apart, but the Louisana one gets farther. Far enough that it requires a sizable force to put it down. The CSA finds out, through a messy trail left behind, that it was engineered by the French. They complain to the British about it. The diplomatic response is "live with it."
The British aren't in a position to respond. French troops are quartered with British troops and hopelessly intertwined. They use the same joint supply lines. Trying to fight the French would would be suicide. The British position is to see it through and commence damage control when all is said and done.
The CSA is in an even worse position. They take the British postion and they have the potential to loose several of the States to French ambitions. If they fight them, they will probably loose. The French and British are already sitting in their home territory, in several cases quartered with one another. The French and British can also switch sides and help the Union pacify the South.
Now some of the CSA States could switch back to the Union, but that does come close to guaranteeing victory, nor is it conducive what they had been fighting this war over. They also risk some measure of retribution and distrust if they do switch sides.
Settling their differences with the Union goes from being an inconceivable notion to the most reasonable course of action. Combined, their forces, having the home advantage, have the greatest chance of repelling the French/British. The CSA knows it is going to have to compromise after the war it over, but it is the price that has to be paid.
That is how you get an alliance and Canada and Mexico involved...