The significant American conflict that occurred between Northern and Southern states from 1861 to 1865 is the American Civil War. The war stemmed from escalating tensions surrounding issues such as states' rights, slavery, and economic disparities. The conflict saw key battles, including Gettysburg and Antietam, leading to immense casualties and profound societal changes. The Union’s victory preserved the nation and ultimately led to the abolition of slavery through the 13th Amendment. The Civil War reshaped the United States, influencing its social, political, and economic landscape for generations to come and establishing a foundation for future civil rights movements.