American Civil War
The American Civil War was the first (and so far only) major civil war in American history, running from April 12, 1861, to April 9, 1865. The conflict was the result of a conflict over slavery in the first half of the 19th century in the US between the Northern states -- where slavery was not profitable and had been quietly abolished in the early years of the union -- and the Southern states -- where it was still legal and the economy had become dependent on slave-based agriculture. This culminated with the election of Abraham Lincoln, who managed to unite the various factions that opposed the expansion of slavery into newly colonized areas. Rich slaver owners in the South, fearing that Lincoln would outlaw slavery, encouraged a number of states to break away from the United States of America and form the Confederate States of America. This led to a war with the Northern states, which were referred to as the Union in the war.
The Confederate states eventually lost the war and rejoined the United States of America. The Northern victory was brought about largely due to the greater size of the Northern population (22 million versus 9 million in the CSA, including 3.5 million slaves which could not be militarized) and the superior industrialization of the North over the rural South. The war is infamous for that fact 660,000 soldiers died during the conflict, more Americans than any other war (some of the battles had more American deaths than any war America had previously fought in).   These tragic losses prompted Robert E. Lee to say, "It is well that war is so terrible -- lest we should grow too fond of it." Massed infantry charges were also shown to have become ineffective during the war due to advancements in weapons, such as rifled muskets and canister shot.
In an attempt to avoid looking bad, some people on the Confederate side created the Southern Lost Cause writings, which created Neo Confederatism.