Timeline-191 is the fan-given name to the Great War Multiverse written by author Harry Turtledove. It got its name from the POD where Robert E. Lee recovered his lost "Special Orders 191" during his invasion of the North in 1862, allowing him to crush General McClellan's Army of the Potomac, occupy Philadelphia, and secure independence and international recognition of the Confederate States of America.
The Great War Series
The series first novel, How Few Remain, is set in the 1880s during a second war between the USA and the CSA; the USA loses, and begins preparing for a future war, ala France after the Franco-Prussian War. The Great War series picks up from there, taking place during an alternate World War I which pits the British Empire, French, Russians, and Confederates against Germany, Austria-Hungary, and the USA, along with minor powers on both sides. The USA and Germany ultimately win the war and impose harsh peaces on their enemies.
The American Empire trilogy picks up at the end of the Great War trilogy, and follows characters North and South (as well as Canada and Quebec) going through their peacetime lives against the backdrop of progressivism in the USA and fascism in the CSA. When the Great Depression occurs in the 1930s, the far-right, led by the Freedom Party, wins election after election, ultimately securing the presidency for their leader, Jake Featherston (modeled on Adolf Hitler), an angry bitter veteran who preaches a doctrine of hate against the USA as well as the old order and the blacks (who had launched a leftist rebellion during WWI) in the CSA. The Freedom Party marginalizes political opposition, persecutes blacks to the point of genocide, and re-arms for yet another confrontation with the USA. Ultimately Featherston gets what he wants, and hurls the Confederate Army into Ohio at the end of the trilogy.
World War II in North America breaks out on the first page of the Settling Accounts tetralogy as Confederate bombers strike targets across the USA. Under General Patton, the CSA cuts the USA in two by striking through Ohio toward Lake Erie. The USA refuses to heel, and Featherston, who had hoped on striking such a strong blow that his foes would have no choice but to surrender, is at a loss as to why the USA wouldn't give in. He orders a renewal of the offensive in 1942, which comes to grief at Pittsburgh (modeled on Stalingrad). In the meantime, his "population reduction" program against Confederate blacks picks up pace in death camps across the Deep South, ultimately resulting in the deaths of 6-10 million.
The North smashes the CSA back into its own country, taking Atlanta at the end of 1943 and Savannah in 1944, cutting the CSA in two. By then atomic weapons have been developed and deployed by several powers, including a Confederate deployment in West Philadelphia and US counter-destruction of Newport News and Charleston. Jake Featherston abandons his capital and attempts to flee to Mississippi but is shot dead in central Georgia, effectively ending the war. The CSA ceases to exist, and the USA imposes a harsh occupation of the country.
TL-191 has its pros and cons, and was and still is bitterly debated by fans and haters. It's one of the largest alternate history series in print, and it covers all sorts of angles of an alternate North America. On the other hand, people blast Turtledove for resorting too much to parallels and analogs, such as his depiction of Featherston. They also hit him for repetition and unrealistic ideas. In the end, though, TL-191 is probably one of the more popular alternate history stories in print.