Britain and France would have gone all in on the Confederate States of America and favored them heavily with trade. Britain was wholly dependent on the South for cotton and desperately wanted to trade with the Confederates if it weren't for American threat of war. Shit got dicey. If England and France had gotten their shit together the Confederate States of America would have had millions of British troops in it, with still more in Canada, and British and French blockades of American ports. The United States might have fallen, or at least been reduced to an angry, defiant and violent backwater like Northern Ireland in the Troubles. The Confederate States of America would meanwhile have been an ugly-ass version of Jamaica where slavery was never banned. With a united England and France intent to quell the colonies, Mexico would probably still be French. It's pretty appalling, sure. And the South is nearly always embarrassing. But the stakes were dire.