In the early nineteenth century, the United States tripled its territory through a steady combination of purchases, negotiations, and wars, drawing land from France, Spain, the United Kingdom, and Mexico. With expansion came a deepening crisis. Every new territory forced the question: would it be open to slavery or free? The North, growing rapidly in population and industry, opposed the spread of slavery.

The South, built on agriculture and enslaved labor, fought to protect it. The nation had more than 23 million people in the North and only about 9 million in the South, including roughly 3. 5 million enslaved people. Both sides claimed the Declaration of Independence and the Constitution supported their cause, but neither interpretation could reconcile the other.

The founding fathers had left the nation’s fundamental questions open to interpretation, and Americans argued over citizenship for Black people, the morality of slavery, and even the true intentions of the men who wrote the Constitution. Some sought peaceful answers through compromise, but those compromises were often extreme and bitterly contested. The Compromise of 1850 admitted California as a free state, allowed territories north of the Missouri Compromise line to decide their own status, and imposed a harsh Fugitive Slave Law that required even free states to return escaped enslaved people. Northerners grew increasingly unwilling to accept the institution, and the founding of the Republican Party in 1854, which opposed the spread of slavery, deepened the divide.

Congress became so volatile that members carried firearms, and physical brawls broke out between representatives. The election of Abraham Lincoln in November 1860 as the first Republican president was the breaking point. Lincoln held moderate views and had no immediate plan to abolish slavery where it already existed, but seven slave states did not wait for his inauguration. Before Lincoln took office in March 1861, they declared their secession and formed the Confederacy under President Jefferson Davis.

The war began in April 1861 when South Carolina attacked Fort Sumter after the federal government refused to withdraw its troops. The fort surrendered, and the federal government called on every state to send forces to reclaim it. Four states refused to fight their southern neighbors and joined the Confederacy. Four border states, Maryland, Delaware, Missouri, and Kentucky, opposed both secession and the use of force against the South.

West Virginia later separated from Virginia to become an additional border state. At the start of the war, the South mobilized its population for military service and benefited from skilled commanders, most notably Robert E. Lee. The Union struggled with outdated and ineffective generals.

The North quickly established a naval blockade to cut off southern supply lines. The first major engagement came at Bull Run on July 21, 1861, when Union forces attempted to march on Richmond, the Confederate capital. The battle was chaotic. Many soldiers had no combat experience, the flags of the two sides were so similar that troops struggled to tell friend from foe, and civilians came from nearby Washington to picnic and watch the fighting.

The Confederates repelled the Union forces, and the North entered a grim period of the war. But in the West, a Union general named Ulysses S. Grant began to earn recognition through a series of crucial victories. By February 1862, Union forces had secured central Kentucky and Tennessee and pushed south to seize a vital east-west railroad junction in northern Mississippi.

The fighting produced more than 23,000 casualties on both sides, but the Union emerged victorious. Across the Atlantic, tensions with Great Britain nearly exploded in November 1861 during the Trent Affair, when U. S. forces intercepted Confederate diplomats aboard a British vessel.

The diplomats were released, but European powers continued to threaten intervention, especially since the Union blockade prevented the South from exporting cotton to Europe. Buoyed by recent southern victories, Robert E. Lee assembled his forces for the first time and marched into northern territory, intending to threaten Washington. The campaign was carefully planned, but the plan unraveled when a Union soldier discovered a copy of Lee’s orders wrapped around a cigar.

The Battle of Antietam followed on September 17, 1862, the bloodiest single day in American military history, with approximately 23,000 casualties. The battle ended in a stalemate, and Lee’s invasion of the North was halted for the time being. On January 1, 1863, Lincoln issued the Emancipation Proclamation, declaring freedom for enslaved people in the Confederate states. The proclamation reshaped the war’s purpose.

What had been a struggle to preserve the Union now also became a fight to end slavery. The decision gave the Union cause greater moral legitimacy and made it nearly impossible for France and Great Britain to intervene on behalf of the Confederacy, since both nations had already abolished slavery and would have appeared hypocritical in siding with it. The war reached a critical turning point in July 1863 when Lee launched another attempt to threaten Washington, leading to the Battle of Gettysburg. The Union suffered about 23,000 casualties, and the Confederates about 28,000.

The battle ended in a decisive Union victory. In the West, Grant gained control of the Mississippi River after the siege of Vicksburg. The dual victories at Gettysburg and Vicksburg marked a pivotal moment. They ensured that European powers would not recognize the Confederacy as an independent nation, denying it essential support.

Four months after the fighting at Gettysburg, the Soldiers’ National Cemetery was dedicated on November 19, 1863. Lincoln was not the main speaker that day. Another orator reportedly spoke for two hours before him. But it was Lincoln’s brief address, just 271 words, that endured.

In it, he redefined the meaning of the war, framing the United States as a nation founded on the principle of liberty and the belief that all people are created equal. In 1864, Lincoln appointed Grant, whom he trusted as his foremost commander despite Grant’s well-known fondness for whiskey, as general-in-chief of the Union Army. Grant was ordered to press the Confederacy on multiple fronts. General William Sherman besieged and captured Atlanta, while in the eastern theater Grant and Lee fought a series of brutal battles that produced more than 80,000 casualties within weeks and ended in a standoff near Richmond.

During the 1864 election, Lincoln won reelection with ninety-one percent of the vote. Sherman advanced from Atlanta through Georgia and South Carolina, destroying railroads, farms, and property valued in the billions while liberating enslaved people. In the East, Lee, under mounting pressure, retreated from Grant’s siege, and Richmond, the Confederate capital, was destroyed. Lee surrendered a few weeks later, and the other Confederate generals followed, effectively ending the war.

The 13th Amendment, abolishing slavery and involuntary servitude, passed the Senate on April 8, 1864, and the House of Representatives on January 31, 1865. Lincoln did not live to see it ratified. He was shot by John Wilkes Booth at Ford’s Theater in Washington on April 14, 1865, and died the next day. The amendment was ratified by the necessary number of states on December 6, 1865.

Jefferson Davis was captured, and the South was placed under military occupation. The country faced the enormous task of reconstruction and reconciliation. The war had claimed more than 750,000 lives, making it the deadliest conflict in American history. Soon after, Black Americans were granted citizenship and the right to vote, and families divided by slavery and war were reunited.

For millions of Black Americans, decades of suffering had come to an end, though the struggle against discrimination and for true equality would continue long after, and its effects are still felt in American society today.