The U.S. government entered the Civil War, which began in 1861, primarily to preserve the Union and ultimately to end slavery. The conflict arose from deep-rooted tensions over states' rights and the expansion of slavery into new territories. President Abraham Lincoln, while initially focused on unity, shifted his goals as the war progressed, issuing the Emancipation Proclamation in 1863, which declared the freedom of all enslaved people in Confederate-held territory. The war fundamentally altered the nation, leading to the passage of the 13th Amendment, which abolished slavery, and laid the foundation for future civil rights advancements.