The significant military conflict fought in the U.S. between 1861 and 1865 was the American Civil War. This war was primarily fought over issues related to slavery, state sovereignty, and economic differences between the Northern and Southern states. The conflict pitted the Union, representing the Northern states, against the Confederacy, formed by secessionist Southern states. It resulted in profound social, economic, and political changes, leading to the abolition of slavery and substantial advances in civil rights. The war, one of the bloodiest in American history, left a lasting legacy and shaped the trajectory of the nation’s development.