The Civil War erupted from deep divisions over slavery, states' rights, economic differences between the industrializing North and the agricultural South, and the election of Abraham Lincoln in 1860, which prompted Southern secession. Lincoln led the nation through the war with the goal of preserving the Union, but on January 1, 1863, issued the Emancipation Proclamation, declaring all enslaved people in Confederate states to be free. While the proclamation did not immediately free anyone in practice, it transformed the war's purpose and permitted African Americans to serve in the Union Army. The pivotal Battle of Gettysburg in July 1863, the war's bloodiest engagement with over 50,000 casualties, halted Robert E. Lee's invasion of the North and marked a turning point toward Union victory.
After the war, the era of Reconstruction (1865 to 1877) sought to rebuild the South and integrate formerly enslaved people into American society. Three landmark Reconstruction Amendments transformed the Constitution: the Thirteenth (1865) abolished slavery, the Fourteenth (1868) granted citizenship and equal protection of the laws, and the Fifteenth (1870) prohibited denying the vote based on race. Reconstruction's promise was betrayed when federal troops withdrew under the Compromise of 1877, allowing Southern states to impose Jim Crow laws that mandated racial segregation in public facilities, disenfranchised Black voters, and entrenched second-class citizenship for decades. The Supreme Court's 1896 Plessy v. Ferguson decision upheld the doctrine of "separate but equal," legalizing segregation until Brown v. Board of Education overturned it in 1954. Even earlier, the 1848 Seneca Falls Convention, organized by Elizabeth Cady Stanton and Lucretia Mott, had launched the women's rights movement with its Declaration of Sentiments, beginning a long struggle that would eventually yield the Nineteenth Amendment in 1920.