As the nation grew, more states joined the United States. The debates over the balance of power between free states and slave states grew more heated. The Compromise of 1850 solved the problem for the time being. It allowed California into the union as a free state; established the Fugitive Slave Law of 1850, which allowed the capture of escaped slaved in free states; and allowed New Mexico and Utah to make their own decisions about slavery.

The passing of the Kansas-Nebraska Act in 1854, which allowed those territories to decide for themselves whether to allow slavery, angered many abolitionist groups. Strongly opposed to slavery, the Republican Party was created in 1854. Abraham Lincoln was a member of the new political party. Dred Scott, an enslaved man, asked the Supreme Court for freedom because he lived in a free territory. He was denied. Many Northern leaders, including Abraham Lincoln, did not agree with the court's decision. In 1859, John Brown led a rebellion at a federal building in Harper's Ferry to try to end slavery, but failed. Lincoln ran for senator against Stephen A. Douglas.

When Abraham Lincoln won the election of 1860, many Southern states threatened to leave the Union. Soon afterward, South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas formed a new country, the Confederate States of America, and chose Jefferson Davis as its president.