Sectional politics over slavery in the United States had previously prevented annexation because Texas would have been admitted as a slave state, upsetting the balance of power between Northern free states and Southern slave states. In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory to Oregon, California (also a Mexican territory), and Texas by any means, with the 1845 annexation of Texas furthering that goal. However, the boundary between Texas and Mexico was disputed, with the Republic of Texas and the U.S. asserting it to be the Rio Grande and Mexico claiming it to be the more-northern Nueces River. Polk sent a diplomatic mission to Mexico in an attempt to buy the disputed territory, together with California and everything in between for $25 million (equivalent to $778 million in 2023), an offer the Mexican government refused. Polk then sent a group of 80 soldiers across the disputed territory to the Rio Grande, ignoring Mexican demands to withdraw. Mexican forces interpreted this as an attack and repelled the U.S. forces on April 25, 1846, a move which Polk used to convince the Congress of the United States to declare war.

The effects of the gold rush were substantial. Whole indigenous societies were attacked and pushed off their lands by the gold-seekers, called "forty-niners" (referring to 1849, the peak year for gold rush immigration). Outside of California, the first to arrive were from Oregon, the Sandwich Islands (Hawaii), and Latin America in late 1848. Of the approximately 300,000 people who came to California during the gold rush, about half arrived by sea and half came overland on the California Trail and the California Road; forty-niners often faced substantial hardships on the trip. While most of the newly arrived were Americans, the gold rush attracted thousands from Latin America, Europe, Australia, and China. Agriculture and ranching expanded throughout the state to meet the needs of the settlers. San Francisco grew from a small settlement of about 200 residents in 1846 to a boomtown of about 36,000 by 1852. Roads, churches, schools and other towns were built throughout California. In 1849, a state constitution was written. The new constitution was adopted by referendum vote; the future state's interim first governor and legislature were chosen. In September 1850, California became a state.

Brown's party of 22 was defeated by a company of U.S. Marines, led by First Lieutenant Israel Greene. Ten of the raiders were killed during the raid, seven were tried and executed afterwards, and five escaped. Several of those present at the raid would later be involved in the Civil War: Colonel Robert E. Lee was in overall command of the operation to retake the arsenal. Stonewall Jackson and Jeb Stuart were among the troops guarding the arrested Brown and John Wilkes Booth was a spectator at Brown's execution. John Brown had originally asked Harriet Tubman and Frederick Douglass, both of whom he had met in his transformative years as an abolitionist in Springfield, Massachusetts, to join him in his raid, but Tubman was prevented by illness and Douglass declined, as he believed Brown's plan was suicidal

The raid was extensively covered in the press nationwide—it was the first such national crisis to be publicized using the new electrical telegraph. Reporters were on the first train leaving for Harpers Ferry after news of the raid was received, at 4 p.m. on Monday, October 17. It carried Maryland militia, and parked on the Maryland side of the Harpers Ferry bridge, just 3 miles (4.8 km) east of the town (at the hamlet of Sandy Hook, Maryland). As there were few official messages to send or receive, the telegraph itself was carried on the next train, connected to the telegraph wires that had been cut at the start of the raid, and was "given up to reporters" who "are in force strong as military" By Tuesday morning the telegraph line had been repaired,and there were reporters from The New York Times "and other distant papers".

Born in what was then part of Virginia (now in West Virginia), Jackson received an appointment to the United States Military Academy, graduating in the class of 1846. He served in the United States Army during the Mexican–American War, distinguishing himself at the Battle of Chapultepec. From 1851 to 1861, he taught at the Virginia Military Institute.

When Virginia seceded from the United States in May 1861 after the Battle of Fort Sumter, Jackson joined the Confederate States Army. He distinguished himself commanding a brigade at the First Battle of Bull Run in July, providing crucial reinforcements and beating back a fierce Union assault. Thus Barnard Elliott Bee Jr. compared him to a "stone wall", which became his enduring nickname.

In the final years of the American Civil War and the Reconstruction Era that followed, Congress repeatedly debated the rights of the millions of black freedmen. By 1869, amendments had been passed to abolish slavery and provide citizenship and equal protection under the laws, but the election of Ulysses S. Grant to the presidency in 1868 convinced a majority of Republicans that protecting the franchise of black male voters was important for the party's future. On February 26, 1869, after rejecting more sweeping versions of a suffrage amendment, Republicans proposed a compromise amendment which would ban franchise restrictions on the basis of race, color, or previous servitude. After surviving a difficult ratification fight and opposition from Democrats, the amendment was certified as duly ratified and part of the Constitution on March 30, 1870. According to the Library of Congress, in the House of Representatives 144 Republicans voted to approve the 15th Amendment, with zero Democrats in favor, 39 no votes, and seven abstentions. In the Senate, 33 Republicans voted to approve, again with zero Democrats in favor.