This is quite frankly ridiculous. Public sentiment in Britain was at best lukewarm on the Confederacy, and slavery was widely despised among the political and working classes. Indeed, much of British colonialism in central and eastern Africa was accompanied by the banning of slavery in conquered areas--whether as justification or to assuage guilty consciences is irrelevant. The French were a decaying dictatorship barely able to control Mexico--a country that had less than two decades before been beaten black and blue by a much weaker and less militarized Union, and more importantly there's no way that the French would so much as consider moving in North America without British permission for fear of causing tensions across the Channel right as the Germans were starting to really coalesce against them.
If it's ridiculous, then please cite something. Perhaps you missed it, but to quote from
Blue and Gray Diplomacy: A History of Union and Confederate Foreign Relations by Howard Jones, the Chapter
Antietam and Emancipation -
Second Bull Run encouraged the Palmerston ministry to consider southern separation as the key to stopping a war that the Union must accept as over. In light of their growing desperation, the prime minister and his foreign secretary refused to believe that Washington had any resiliency left. Palmerston and Russell thus linked either approval or rejection of mediation by the Union with an admission to independence that, by definition, pointed to ultimate recognition of a Confederate nation. Yet the Lincoln administration continued to renounce mediation as an unwarranted interference in American affairs that would prolong the war by holding out the prospect of southern recognition. The British again ignored the Union’s warnings against any kind of intervention and insisted that they sought only to bring the two warring parties to the peace table. But the White House correctly suspected that mediation marked the first step in a process that as a matter of course would lead to a foreign acclamation of separation and then, finally, to recognition. What other outcome could there be once the Union refused a public offer of mediation from one or more European powers that claimed only to want the war to end? Recognition, the Union realized, would open the Confederacy to commercial and even military agreements, making the European nations virtual if not actual allies of the new nation. With the welfare of one or more continental powers then tied to the Confederacy, the peacemakers would be under enormous pressure to use force to end the conflict.
These events might have played out in the autumn of 1862, had not Confederate general Robert E. Lee followed his victory at Second Bull Run with a raid into Maryland.
As for the French in Mexico:
"By the fall of 1864 the French army reached the northern border with Texas and was able to benefit from the lucrative trade with the embattled Confederate States in the civil war north of the Rio Grande. Also, in the far south, Bazaine defeated and forced the surrender of 8,000 republican troops under Porfirio Diaz in Oaxaca in early 1865. It was the last major republican force still in the field though it had little to no contact with Juarez himself. The fugitive president was, by that time, living constantly on the run in the northern reaches of Chihuahua just south of the Arizona border."
Actual full-on UK intervention is a near-impossibility. The Trent affair--a blatant insult to Britain of the highest order--would certainly have led to war if there was any serious intent or will on the part of the British people or political class to intervene on the side of the slavocrats' rebellion. Also, though Palmerston was reputed to be wary of American power, he was also an opponent of slavery, and Trent proved unequivocally that he was unwilling to seize a pretext on a silver platter to intervene.
Actually Palmerston did; it was only the intervention of the Prince Consort that prevented war by toning down the British ultimatum. As it existed, it was acknowledged it was likely to start a war otherwise.
There is simply no realistic way that the CSA could do better than OTL without significant changes to the international situation going back prior to the War of Southern Aggression. Perhaps a weakening of Prussia in the early 19th century leading to a more stable Continent divided between French and Austrian spheres of influence, and a Britain led by a Prime Minister with fewer scruples? But not with the OTL situation and political figures.
I've already cited numerous examples they could, backed up by the historians. You're entitled to your opinion, of course, but I am likewise able to say it's just not supported by the historical record. Numerous possibilities of more Confederate victories exist and even James McPherson himself crafted such a scenario.
