The American Civil War began on April 12, 1861, when Confederate forces opened fire on the Union-held Fort Sumter. Fort Sumter is located in the middle of the harbor of Charleston, South Carolina.^[26] Its status had been contentious for months. Outgoing President Buchanan had dithered in reinforcing the Union garrison in the harbor, which was under command of Major Robert Anderson. Anderson took matters into his own hands and on December 26, 1860, under the cover of darkness, sailed the garrison from the poorly placed Fort Moultrie to the stalwart island Fort Sumter.^[27] Anderson's actions catapulted him to hero status in the North. An attempt to resupply the fort on January 9, 1861, failed and nearly started the war then and there. But an informal truce held.^[28] On March 5, the newly sworn in Lincoln was informed that the Fort was running low on supplies.^[29]

Fort Sumter proved to be one of the main challenges of the new Lincoln administration.^[29] Back-channel dealing by Secretary of State Seward with the Confederates undermined Lincoln's decision-making; Seward wanted to pull out of the fort.^[30] But a firm hand by Lincoln tamed Seward, and Seward became one of Lincoln's staunchest allies. Lincoln ultimately decided that holding the fort, which would require reinforcing it, was the only workable option. Thus, on April 6, Lincoln informed the Governor of South Carolina that a ship with food but no ammunition would attempt to supply the Fort. Historian McPherson describes this win-win approach as "the first sign of the mastery that would mark Lincoln's presidency"; the Union would win if it could resupply and hold onto the Fort, and the South would be the aggressor if it opened fire on an unarmed ship supplying starving men.^[31] An April 9 Confederate cabinet meeting resulted in President Davis's ordering General P. G. T. Beauregard to take the Fort before supplies could reach it.^[32]

At 4:30 am on April 12, Confederate forces fired the first of 4,000 shells at the Fort; it fell the next day. The loss of Fort Sumter lit a patriotic fire under the North.^[33] On April 15, Lincoln called on the states to field 75,000 volunteer troops for 90 days; impassioned Union states met the quotas quickly.^[34] On May 3, 1861, Lincoln called for an additional 42,000 volunteers for a period of three years.^[35] Shortly after this, Virginia, Tennessee, Arkansas, and North Carolina seceded and joined the Confederacy. To reward Virginia, the Confederate capital was moved to Richmond.^[36]