The movie Gettysburg is centred around Colonel Chamberlain, considered by some to have changed the course of history. It is interesting to speculate on this "alternative" history.
Chamberlain defended the left Union flank at Little Round Top, exhausted of ammunition. His decision to charge the Confederates led to their surrender and forced Lee to consider a direct assault from the West on Day 3, something he despised doing since he had no intelligence on the exact strength of the Union forces. As it turned out, forces that came during Day 2 were available in Day 3 for the rout of the Confederates and the beginning of the end for the Northern campaign and the war.
But what if Chamberlain had faltered? What if he had opted to stand fast and weather another Confederate wave? There is a good chance that the left flank would have been compromised and that the Confederates would have poured around the end of the Union line. From here, Little Round Top would have fallen and intelligence back to Longstreet would have reinforced the resolve to take the Round Tops, both Little and Big.
It would have been a short job to dispatch the Union forces, short on ammunition all round. After the fall of Little and Big round top, Union forces parked on the ridge south of Gettysburg (Cemetery Ridge) would have been highly vulnerable, especially considering that the Confederates could have come out of the dense vegetation of Big Round Top in the kind 'guerrilla' style that Confederates were good at.
Without the high ground of Cemetery Ridge, Union forces would almost certainly have fallen, since reinforcements from the south would have been unable to reach them and Confederate forces would have been reinforced via Big Top. In all, the Union forces would effectively be surrounded.
Another decisive victory to Lee, this time in Pennsylvania (allowing replenishment of Southern forces) and close enough to Washington to worry its inhabitants, would force Lincoln's hand in giving in to a war becoming highly unpopular in the North because of the substantial casualties.
Almost certainly Lincoln would have lost a lot of face and would have entered into an uneasy truce with the CSA.
This would have resulted in many minor skirmishes for decades while the line between North and South was decided, creating a stronger bond between Britain and the CSA and alientating the North, possibly creating a bond with an emerging Germany.
Emboldened by the success of secession, California and Texas would have led a charge towards independence, both as Anglo-Latin countries, emerging in the modern era countries akin to middle and south American republics. Likewise, some northern states who had considered secession, such as Massachusetts, might have gone independent from a USA somewhat weakened. Thus, North America would emerge as a conglomerate of states equivalent to South America, with no strong USA 'giant'.
Without USA as we know it and possibly with a pact between the USA and Germany aligned with a pact between the CSA and Britain, imperial Germany would have presented a far more formidable opponent in WW1 and almost certainly have won the war at sea with U-Boats.
Additionally, the might of the industrial north would have been severely curtailed without access to cheap black labour and the inevitable industrialisation of the South would have led to the 'emancipation' of blacks in an apartheid state similar to South Africa. This would have balanced the North South power and extended the conflict to achieve supremacy.
Thence, imperial ambitions throughout the world would have been entirely different. Japan would have been unbridled in the Pacific, especially in China, eventually formed a Japanese empire, slowly making its way into Northern Australia and across into India by the 1930s and decaying rapidly later in the mid 1900s due to economic restraints of a feudal empire. Germany would have dominated Europe and its empire would eventually fail, once again because of a pre-modern industrial structure. The Nazis and Hitler would have been unknown. Oppenheimer would have developed nuclear capacity for Germany. The Middle East would have been annexed by Germany and Arab governors appointed in most Arab countries. Israel would never have existed.
An uncomfortable Federal Republic of Europe (Bundes Republik Europa) would have emerged, with an independence movement in Britain finally overthrowing the invaders by 1960. The profligate consumerism of the USA would never have emerged, as the economies of the CSA and USA economies were drained in conflict and refugees who injected the entrepreneurial spirit into USA would not have bothered to journey to the USA.
Also, by 1960, the arms race between the USSR and BRE would have escalated into a hot war, with nuclear elements and obliteration of large parts of both Europe and USSR, providing an opportunity for the CSA and USA to call a truce and form, along with most of North and South America, the American Economic Union with their first mission being the re-building of Europe.
