I'm a big fan of the "alternative history" sub-genre of science fiction which explores the past and subsequent future in a a 'what if...?' motif. Harry Turtledove is without question the one great master of this craft. My favorites of his are the "Invasion" series which posit an invasion of intelligent lizards of this planet occurring during World War II. He creates complex characters of ordinary people in extraordinary situations, sometimes interacting with the great and near great. In the Invasion books, a regular guy named Sam who's a minor league baseball player is the protagonist caught up in the invasion of downstate Illinois and thereby entering military service and becoming the "go to" guy for the humans who run things as an expert on the lizards from his interactions with POWs and, if you can believe it, as a foster father of infant lizards. The heroes and villians of the era of the era are there, too: Soviet Foreign Minister Molotov, Nazi bigwig Heinrich Himmler, Polish Jewish resistence leader Mordecai Ancelwicz, and more. It explores how events unfold and how they affect regular people, mostly Sam and those close to him. A rival sub-commander of the lizard leader switches sides after a failed coup and becomes one of Sam's crew. My favorite moment is when the lizards learn from Molotov of the Bolshiveks' execution of Tsar Nicholas and the royal family and are horrified, because they, like the Japanese back then, revered their Emperor and that his occurred leaves them going, "You know, maybe we overreached here..." (They had scouted Earth back when Knighthood was in bloom and figured we'd be easy pickings, but them found out how far we'd avanced, and were like, "Oh, shit!")
Another Turthedove opus I love is "The Two Georges" concerning the late 19th Century theft of a huge painting of an accord between King George III and George Washington, which resulted in us entering a commonwealth status like Canada did. He co-wrote that one with Richard Dreyfus (yes, the one in the movies!).
My first exposure to such things was a paperback popular in the early 1960s at the time of the Civil War Centennial called "If the South won the Civil War" which supposed they had and what would happen. Long story short, Texas split from the Confederacy (well, we had been a republic just fifteen years before Fort Sumter) but eventually slavery was ended amiably and the three Republics reunited.
These days, some of the loudest mouths on the far right are all "there's gonna be a civil war again - just you watch!" It's certainly not likely, but, hey, anything's possible. So below the squiggly, here's my concept of how such a thing could happen, if it could. Keep in mind, it's not my desire for such a thing to occur, but, I despair of the neverending culture wars, and I wanted to tell a story how the thing could come to pass and then come to an end, so here goes.