The major war fought between 1861 and 1865 in the U.S. was the American Civil War. This conflict arose primarily from tensions over slavery and states' rights, leading to the secession of several Southern states from the Union. The war pitted the Northern states (Union) against the Southern states (Confederacy), resulting in significant battles such as Gettysburg and Antietam. The war resulted in the preservation of the Union, the abolition of slavery with the 13th Amendment, and profound social, political, and economic changes. Its legacy continues to shape American history and discussions on civil rights.