Here we have an alternate history story from the book, "Alternate Presidents". The short story is called, "How the South Preserved the Union" by Ralph Roberts. Out of all the stories so far, this one is my favorite. The scenario is that, soon after their Inauguration in 1849, both President Zachary Taylor and Vice-President Millard Fillmore were killed in a carriage accident, resulting in President Pro Tempore, David Rice Atchison being sworn in as President. Because of Atchison was very pro slavery, a faction of Abolitionists began to form in New England, calling for secession. The story is told through the point of view of Sam Garrison, who is appoint "National Security Advider" by President Atchison, who recalls this Civil War on a train ride back to his home in Missouri.
Sam Garrison is sent to the of Elba in the State of New York where he meets the Abolitionist John Brown and Senator Daniel Webster to talk them out of secession. The latter will later become the President of the newly formed "Confederacy of North America" with the former the Commander of the Army. Despite Garrison's efforts, the State of Massachusetts secedes, resulting in the Second Battle of Lexington and Concord between the U.S. Army and the "New Minutemen", resulting in the latter being the victor. The New Minutemen's victory causes the rest of New England to secede along with the States of New York, Pennsylvania, and New Jersey. It is said that Ohio, Indiana, and Michigan followed suit but it is not confirmed.
In 1852, the United States suffers many defeats at the hands of the Confederate Army led by John Brown. What gives the Confederacy an advantage is through numbers, the amount of experienced veterans of the Mexican American War, and that the weapon manufacturers Colt and Srpingfield are in Confederate hands, as they are located in Connecticut and Massachusetts, respectively. After the U.S. Army's defeat at the Battle of Gettysburg, Secretary Henry Clay proposes that the United States recruit Slaves work in the factories to produce weapons, allowing more men to be drafted into the Army. This proposal was followed by a suggestion by John C. Calhoun of South Carolina to put slaves into uniform and sent to the front as well. Both of these proposals were accepted and passed. We next see our characters in Richmond a year later as Washington is captured by the Confederates. The plan to arm slaves and send to the front and put slaves to work in the factories wasn't working out as planned. The Confederates seem to have the upper hand as they manage to burn down Atlanta, capture New Orleans, and take control of Delaware, Maryland, and Northern Virginia. As a last resort, President Atchison enforces an "Emancipation Proclamation", promising that any save who is in the military and working the factories will be guaranteed their freedom, which ultimately works. The proclamation is a success, which creates a large boost in moral among the slaves, or former slaves at this point. The U.S. Army under General Winfield Scott began scoring victory after victory against John Brown and the Confederates. What really helped the United States was an advance in military technology, such as new artillery pieces, the creation of a repeating rifle by an unnamed slave, and the creation of a new Army Corps called the "Air Corps" which consists of Hot Air Balloons and dropping bombs down on the enemy. By 1855, the last of the Confederate forces had surrendered, Daniel Webster had passed aware, and John Brown was hanged. Later in 1861, President Stephen A. Douglas abolished slavery in the United States forever.
Now here are some interesting things in from book:
1. The "Civil War" in our time line is written as a Dime Novel, of which our protagonists finds on a train in the 1880's. He found the novel to be rather dumb, which leads him to recount war he had known.
2. Abraham Lincoln is mentioned in the story. Garrison didn't think highly of him as he scoffed at the idea of being president after reading the previously mentioned Dime Novel. In this time line's 1865, Lincoln was shot in a tavern by an actor named John Wilkes Booth over some theater tickets.
3. George Custer survives the Battle of Little Big Horn and becomes President. Buffalo Bill is said to be a politician in this time line as well, but it isn't confirmed if he was President or not.
4. Sam Garrison later becomes President Atchison's Secretary of State during his second term and later becomes Senator of Missouri in 1860.
Also, I would to address the wiki box above. It doesn't state the specific date when Taylor and Fillmore die, so I chose the mostly likely date, despite the book saying that Sam Garrison is recalled to Washington D.C. in February. I assume this to be an oversight. In regards to Atchison's Vice President, it never really states who he is, so I chose the most likely as well.
Lastly, a fun fact, David Rice Atchison was actually President...
...for one day. As the 4th of March, the standard Inauguration Day before 1941 landed on a Sunday, so the date was moved to Monday on the 4th. As President Pro Tempore, Atchison was the acting President for the day until Taylor could be sworn in.