One interesting scenario here might be whether the southern envoys happen to take a French rather than a British ship from Cuba and are intercepted by Wilkes as OTL? Napoleon III is less likely to moderate a French reply as Prince Albert did the British and Washington may think France less likely to go to war/less of a threat or be angered by the tone of the French response and you could see France at war with the union.
At worst Britain tries to mediate and ends up at war as well. [Since the French have a strong case for compensation and an apology from the union and its an issue, freedom of the seas, that is very important for Britain].
Failing that French doesn't have as powerful a navy as Britain and lacks nearby bases so any blockade will be weaker. Plus there's no potential northern border to tie down union troops. However it can still lift the northern blockade and smash a good chunk of the union fleet if things go badly for the union. Even a limited blockade will have a a big effect on the union, especially if Britain recognises it in terms of accepting the banning of war materials, including guns and saltpetre for the union. It will still free up a fair amount of southern forces and also their overseas trade and coastal traffic, giving them a big boost. Also a couple of trained and equipped French corps fighting alongside Lee could greatly complicate the union mission.
Also such a clash is likely to be rapidly followed by a formal French recognition of the south, which could be followed by other nations, which would pose a problem for the union as I think it threatened it would see any such recognition as a casus belli by the union. If so either it has to back down or it could find itself technically at war with a number of countries.
Such a situation might save Mexico from armed French intervention because its too busy in the USCW but that's likely to be small satisfaction for the union.
In this circumstance this could be a pretty long and bloody war with I suspect ultimately the union having to recognise the independence of the south but possibly holding some border areas of the seceded states. Whether France would be too weakened to face up to Prussia in 187o or learn enough it does markedly better or anywhere in between there could be substantial butterflies in world history well beyond N America.
At worst Britain tries to mediate and ends up at war as well. [Since the French have a strong case for compensation and an apology from the union and its an issue, freedom of the seas, that is very important for Britain].
Failing that French doesn't have as powerful a navy as Britain and lacks nearby bases so any blockade will be weaker. Plus there's no potential northern border to tie down union troops. However it can still lift the northern blockade and smash a good chunk of the union fleet if things go badly for the union. Even a limited blockade will have a a big effect on the union, especially if Britain recognises it in terms of accepting the banning of war materials, including guns and saltpetre for the union. It will still free up a fair amount of southern forces and also their overseas trade and coastal traffic, giving them a big boost. Also a couple of trained and equipped French corps fighting alongside Lee could greatly complicate the union mission.
Also such a clash is likely to be rapidly followed by a formal French recognition of the south, which could be followed by other nations, which would pose a problem for the union as I think it threatened it would see any such recognition as a casus belli by the union. If so either it has to back down or it could find itself technically at war with a number of countries.
Such a situation might save Mexico from armed French intervention because its too busy in the USCW but that's likely to be small satisfaction for the union.
In this circumstance this could be a pretty long and bloody war with I suspect ultimately the union having to recognise the independence of the south but possibly holding some border areas of the seceded states. Whether France would be too weakened to face up to Prussia in 187o or learn enough it does markedly better or anywhere in between there could be substantial butterflies in world history well beyond N America.