The Civil War in the United States was a conflict that took place from 1861 to 1865 between the Northern states (known as the Union) and the Southern states that seceded from the Union and formed the Confederacy. The war was primarily fought over issues of slavery, states' rights, and the preservation of the Union. It was the deadliest war in American history, resulting in the death of approximately 620,000 soldiers and an unknown number of civilian casualties. The war ended with the defeat of the Confederate states and the abolition of slavery throughout the United States.
The Civil War was a pivotal moment in American history that not only resolved the question of slavery but also significantly altered the nature of federalism in the United States. The victory of the Union reaffirmed the primacy of the federal government over the states and set the stage for Reconstruction, which aimed to rebuild the South and integrate formerly enslaved people into American society as citizens with equal rights.