The problem with the South winning that war is that overall American military power would have been neutralized. Instead of one vast Arsenal of Democracy, you would have had two great military powers that would have generally cancelled each other out, much like a husband and a wife voting for different political candidates.
As a single power, the combined U.S. was able to overcome the great European stalemate of 1917 as well as defeat the vast warring machines of Japan, Italy, and Germany after 1941.
But a split U.S./Confederacy would have been unable to enforce the Monroe Doctrine in South America (where war would have *again* broken out as European powers allied with opposing USA/CSA factions would have gone to war over colonies there), much less been the decisive factor in WW1 and WW2.
The inevitable result would have been that Europe would have controlled global politics for another century or two, that wars would have been more frequent(Africa and South America and the Middle East would have been constantly warred on for their minerals by various perverse alignments of European and CSA/USA factions), and that poverty would have been higher while population rates lower (due to increased deaths in war and from added poverty).
Decisive victories against Germany would have been out of the question. You'd still have had the Holocaust, only led by the Kaiser due to social pressures rather than by a Hitler type.
You would *not* have Freedom of the Seas. Global movements of goods and people would be highly restricted due to the lack of an all-powerful Navy (this single factor is probably the most overlooked American contribution to current global prosperity).
In short, a split USA/CSA would have kept the world's military and economic powers more closely matched.
A single, all-powerful USA, however, has always tipped significant military and economic conflicts in a single direction, shortening wars and leading to greater economic advancements.
But a split USA/CSA would have turned the Hundred Years war into the Millenium War, if not longer.
And all of this change in power was due to a single horseman. In what we now know as the battle of Antietam, McClellan's forces intercepted Lee's Special Order 191...which were written on parchment and wrapped around 3 cigars...and subsequently dropped by the courier as he rode from Lee's tent to Jackson's camp.
SO 191 detailed Lee's brilliant plan to use most of his forces...but under Jackson's command. With McClellan expecting Lee's own Army to have the superior Southern forces, Jackson's flanking attack could be expected to be a mere diversion...so Lee's plan to have Jackson's attack be the real blow would catch the Union by surprise.
But McClellan had possession of this great plan. However, possession of this plan was quite a dilema. If the SO 191 was a ruse, then an immediate headfirst attack into Lee's full forces would be a disaster for the Union. Did Lee arrange to drop SO 191 on purpose in order to trick McClellan, the Union staff debated?!
On the other hand, if SO 191 was real, then the Union Army could not simply wait in place for Jackson to attack it from one direction and Lee to attack it from the other, framed as it was by the river. Likewise, if SO 191 was real, then attacking Jackson's forces immediately would likewise be a disaster for the Union Army.
So faced with such a delemma, McClellan did the only smart thing (i.e. the low risk option); he advanced only meekly at Lee's forces, forcing a fight with what turned out to be Lee's greatly weakened Army, and thereby moving out of the line of attack by Jackson.
Thus, the September 17, 1862 battle of Antietam is forever viewed as a military "tie" in which McClellan failed to wipe Lee out, even though McClellan's move was a brilliant way to avoid disaster and buy the Union time to get out of its predicament.
...And it was made possible because a single horseman dropped a single piece of paper wrapped around 3 cigars, which were found by 3 illiterate Union soldiers who almost didn't bother to have an officer read what was written therein.
That's how close the world came to perpetual global war among closely matched nations.
The AWB Has Expired - Gun Owners Have Won Again For All Americans!