American Civil War: Lincoln's victory in the presidential election of 1860 triggered South Carolina's declaration of secession from the Union. By February 1861, six more Southern states made similar declarations. On February 7, the seven states adopted a provisional constitution for the Confederate States of America and established their temporary capital at Montgomery, Alabama. A pre-war February Peace Conference of 1861 met in Washington in a failed attempt at resolving the crisis. The remaining eight slave states rejected pleas to join the Confederacy. Confederate forces seized most of the federal forts within their boundaries. President Buchanan protested but made no military response apart from a failed attempt to resupply Fort Sumter using the ship Star of the West, which was fired upon by South Carolina forces and turned back before it reached the fort. However, governors in Massachusetts, New York, and Pennsylvania quietly began buying weapons and training militia units.

On March 4, 1861, Abraham Lincoln was sworn in as President. In his inaugural address, he argued that the Constitution was a more perfect union than the earlier Articles of Confederation and Perpetual Union, that it was a binding contract, and called any secession "legally void". He stated he had no intent to invade Southern states, nor did he intend to end slavery where it existed, but that he would use force to maintain possession of federal property. His speech closed with a plea for restoration of the bonds of union.

The South sent delegations to Washington and offered to pay for the federal properties and enter into a peace treaty with the United States. Lincoln rejected any negotiations with Confederate agents because the Confederacy was not a legitimate government, and that making any treaty with it would be tantamount to recognition of it as a sovereign government. However, Secretary of State William Seward engaged in unauthorized and indirect negotiations that failed.

Fort Monroe in Virginia, Fort Sumter in Charleston, South Carolina, and Fort Pickens, Fort Jefferson, and Fort Taylor, all in Florida, were the remaining Union-held forts in the Confederacy, and Lincoln was determined to hold them all. Under orders from Confederate President Jefferson Davis, troops controlled by the Confederate government under P. G. T. Beauregard bombarded Fort Sumter on April 12, forcing its capitulation. Northerners rallied behind Lincoln's call for all the states to send troops to recapture the forts and to preserve the Union, citing presidential powers given by the Militia Acts of 1792. With the scale of the rebellion apparently small so far, Lincoln called for 75,000 volunteers for 90 days. For months before that, several Northern governors had discreetly readied their state militias; they began to move forces the next day. Liberty Arsenal in Liberty, Missouri was seized eight days after Fort Sumter.

Four states in the upper South (Tennessee, Arkansas, North Carolina, and Virginia), which had repeatedly rejected Confederate overtures, now refused to send forces against their neighbors, declared their secession, and joined the Confederacy. To reward Virginia, the Confederate capital was moved to Richmond. The city was the symbol of the Confederacy. Richmond was in a highly vulnerable location at the end of a tortuous Confederate supply line. Although Richmond was heavily fortified, supplies for the city would be reduced by Sherman's capture of Atlanta and cut off almost entirely when Grant besieged Petersburg and its railroads that supplied the Southern capital.