From
Up With the Cross: The Tragic History of the Confederate States of America, by Albert Samuels:
Later generations of historians universally agree on the ultimate cause of the temporary collapse of the United States of America into two separate states in the 19th and part of the 20th Century. Namely that in terms of US expansion up to the Southron War of Independence the United States had expanded too much, too far, too fast. However while this is held to be the cause of the war, the war itself produced a split of the United States on a suitably named river in terms of the long and horrid legacy that it would lead to. The road to the collapse of the old United States of Yorktown and Chapultepec was paved on the suitably ghastly named area known as Chickamauga.
By a curious irony the Chickamauga region itself had been named after a secessionist movement within the Cherokee nation, a movement whose war within the Cherokee brought ruin and death to the Cherokee, and the Cherokee being bowed under by a harsher, more oppressive future. More prophetically the name Chickamauga itself in Cherokee means "River of Death", and a river of death it indeed proved. So it was that the most suitable place for the dark future that awaited the United States would come with the comedy of errors that produced the victory of the Southron armies in the Battle of Chickamauga. Waged for two terrible days, the battle was won by an overpowering strike by the troops of General James Longstreet, a strike that led to rolling up the ill-fated Army of the Cumberland and trapping it within the walls of Chattanooga.
It would be a worse fate, however, that befell this army when as the Siege had become truly perilous a fatal accident involving a spooked horse ended the career of the most brilliant of the Union generals, setting in motion the disaster of Chattanooga, which would be termed the graveyard of the Union. In the aftermath of Bragg's victory, won by the hardest, and with his army immobile on the relatively towering peaks of Lookout Mountain and Missionary Ridge, Ulysses S. Grant had been riding his horse near a train when the horse was spooked by its passage. The horse reared and fell on General Grant, inflicting fatal internal injuries that led to Grant's passing, delirious, five days later.
This, however, proved only the first stage in the ultimate disaster the Union faced, and the moment when the cause of the unified states of America began to become henceforth and forever lost, damning a continent to upheaval it could never have foreseen.......
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Edit: This is my intended counterpart/parallel to Up With the Star, where another small incident produces a great and terrible set of ripples across the course of history. In keeping with a theme this one is named after the Confederate version of The Battle Cry of Freedom, and in both universes there's a book with the same title as the TL.