During Buchanan's inaugural address, he asserted that the slavery issue was about to be decided by the Supreme Court, ending the sectional differences in the United States. He was referring to the Dred Scott court case.

The court decided that slaves should be considered property and further that Congress had no right to exclude slavery in the territories. This would have wide-reaching effects in terms of how to deal with fugitive slaves. Buchanan felt that this case proved the constitutionality of slavery, but he was incorrect in his belief that this ruling would end sectional strife — it only fanned the flames that would erupt by the end of his term in office.

John Brown's Raid

One event that polarized the nation was John Brown's Raid on a federal armory in Harper's Ferry, Virginia, in October 1859. Brown led eighteen men — five of whom were African American — to seize the armory. He believed that his actions would lead to an uprising of African Americans that in turn would result in a war against slavery. President Buchanan sent Robert E. Lee and the U.S. Marines to stop the raiders. Brown was captured and stood trial for murder, treason, and conspiring with slaves to rebel. He was sentenced to death and hanged in December 1859. Southerners saw this rebellion as one more example of the North plotting to end slavery. Northerners, while disagreeing with the way Brown acted, agreed with his ultimate goal of ending slavery.

Kansas and the Lecompton Constitution

According to the Kansas-Nebraska Act passed under Franklin Pierce, popular sovereignty was to determine whether Kansas was a slave or free state. A number of constitutions were proposed for the new state, the most famous of which was the Lecompton Constitution, which would have made it a slave state. Believing that slavery was constitutional, Buchanan fought for the acceptance of the Lecompton Constitution using many means, including offering cash and office appointments in exchange for votes. The Lecompton Constitution reached a deadlock in Congress and was sent back to the territory to be voted on in a general election by Kansas residents, who overwhelmingly defeated it. The effect of this incident was the split of the Democratic party over the Constitution into northerners and southerners. Kansas did finally enter the Union in 1861 as a free state.

Secession

Buchanan had no plans to run for reelection. With the Democratic party divided, the Republican candidate Abraham Lincoln was able to win the election in November 1860. Upon Lincoln's election, seven states seceded from the Union to form the Confederate States of America. Buchanan did not believe that the federal government could force a state to remain in the Union and therefore did nothing. In fact, two incidents occurred that demonstrated his willingness to compromise to avoid Civil War.

After the secession of Florida, federal troops stationed in Pensacola moved into Fort Pickens to consolidate their forces. Buchanan sent supplies and troops by ship to reinforce the fort. However, when they arrived they only sent the provisions ashore. Buchanan had made a truce with Florida that no further troops would be stationed at the fort unless it was fired on.

In Charleston, South Carolina, troops moved off the mainland to occupy Fort Sumter. Similarly, provisions and troops were sent to reinforce the federal forces. However, confederate forces fired on the ship carrying the troops and it was forced to retreat, but Buchanan ignored this action. President Buchanan left office with the Union divided.

According to a survey of presidential historians conducted by the University of Louisville, Buchanan's failure to oppose the secession of the states was the number one presidential blunder in America's history.