The historical event I have selected is the recovery of Lee’s Special Order 191 by a Union soldier during the American Civil War. The order detailed Lee’s intentions and how he was splitting his forces while invading Maryland and Pennsylvania. The order was intended to be destroyed (it was found wrapped around several cigars), but instead was found and relayed to George McClellan, commander of the Union Army of the Potomac. McClellan had previously been outmaneuvered and outfought by Lee’s Army of Northern Virginia multiple times. However, with this order he was able to predict Lee’s movements, and forestalled the invasion of the North at the Battle of Antietam. Many historians believe that McClellen, a notoriously over-cautious and slow-moving general, could have taken greater advantage of the order. Antietam was a very bloody battle, with heavy casualties on both sides. Lee’s army did retreat, but McClellan, fearing a trap, refused to pursue, despite the insistence of President Lincoln. Hindsight shows that if he had pressed his advantage, the Army of Northern Virginia was not in good shape, and could have been destroyed or severely damaged. A few days later, Lincoln removed McClellan from command for failing to take full advantage of his intelligence.
However, the Battle of Antietam did allow Lincoln to make the Emancipation Proclamation. This was crucial, because the President’s advisors had convinced him to delay the announcement until after a Union victory, so as to not seem like a move of desperation. The result of the Emancipation Proclamation was that France and Britain could not convincingly recognize the Confederacy as a legitimate nation, due to slavery now being a central issue of the war.
Harry Turtledove, the foremost contemporary alternate history author, used Special Order 191 as the point of divergence for his epic alternate history series Southern Victory, where the pertinent copy of Special Order 191 is in fact destroyed. I would propose a similar change as part of a ripple effect from Babbage’s Difference Engines becoming widespread and well-used.
Had Difference Engines been finished, used, and proved helpful enough for common use, the technology of the period leading up to 1860 could have been wildly affected. Babbage would have continued to be prominent, and it seems reasonable to assume that this other ideas, inventions and interests would have become more important among the scientists and engineers of his day. In addition to that, if the Difference Engine succeeded, it’s likely that other mechnical computation devices would have been invented in a similar sense to the electro-mechanical devices that began to flourish after Hollerith’s initial success in the 1890s.
One field that would have been the key beneficiary of these advances would have been cryptography. By World War I, military cryptography was commonplace; however, Special Order 191 was not encrypted, which allowed the Union army to quickly realize its importance, forward it up the chain of command, and understand it. Had cryptography spread to the Confederacy’s armed forces, it would have at the very least taken the Union some time to decrypt the order, were it even realized as important at all by the corporal who found it.
In actual history, McClellan’s deficiencies as a commander were such that even with fantastic military intelligence, he was only able to fight Lee to a standstill, barely enough of a success to allow the Emancipation Proclamation to go out. Without Special Order 191 in hand, it seems more than likely that Lee would have once again humiliated Union armed forces, this time on their own soil, and possibly given the Confederacy enough of an advantage to win the war. Had the CSA’s advantage after Lee’s Maryland campaign been seen as sufficient, Britain and France would likely have recognized the South and broken the blockade to restore the flow of cotton exports and to hurt the USA.
Such a vast change in the power balance on the American continent would have had vast consequences. Certainly, Britain and France would have been enemies of the United States, rather than eventual allies, due to their effective alliance with the CSA. Furthermore, assuming German unification proceeded as in actual history, the USA and Germany could well have applied the principle of “the enemy of my enemy is my friend”, and found common ground in the later 1800s, maybe even in World War I. Had the United States never entered World War I against the Central Powers, but rather been tied down by a war at home (or at least the prospect of being counterbalanced by the Confederate States), the entire 20th century would look completely different. Everything from German backlash to the Treaty of Versailles and the dismantling of the Ottoman Empire to the October Revolution in Russia could have had vastly different outcomes, with a German-Union alliance, and an independent Confederate States of America.
The farther one moves forward from Special Order 191, the greater the implications become. With simple knowledge of encryption, and perhaps even something as simple as a substitution or rotation cipher, the importance of Special Order 191 or at least its meaning would have never been realized (or at least not soon enough). That paper, wrapped around cigars, is one of the hinges upon which history has turned, and with the technological advances that could have been perpetrated by the Difference Engine, history could have turned in a very different direction.