Shortly after Lincoln's inauguration, South Carolina decided to suspend all sales of goods and merchandise to the Union troops who occupied Fort Sumter. This put Lincoln squarely in the same predicament that Buchanan had faced. Union forces were sorely in need of supplies. To send in relief might appear to be an act of aggression, but to withdraw troops would be tantamount to capitulation, and a recognition of Confederate sovereignty.
Under increasing pressure from the federal forces at Sumter, and ignoring the more cautious wishes of his cabinet, Lincoln decided to send supplies to Charleston in the first week of April. When the federal relief expedition arrived at Fort Sumter on April 12, Confederate forces opened fire, and the undermanned and unprepared federal troops were forced to surrender the next day. With this, the war was on.
At the time of this insurrection, the combined forces of the United States equaled a paltry 16,000. Most of these were holding positions on the frontier in an attempt to fight back hostile natives. Now, with a civil war in effect, Lincoln found his forces halved, with the need for numbers raised exponentially. In order to support a weakened federal force, Lincoln made an appeal to the state governors on April 15, with hopes of gaining 75,000 enlisted men. With the period of service set at one hundred days, the forces originally fielded far more volunteers than the government was capable of outfitting. Such are the heady early days of war.
In reinforcing Fort Sumter, and in marshalling federal troops to prepare for battle with the Confederacy, Lincoln had single-handedly made an effective declaration of war. He himself had vehemently opposed such a sweeping step as a member of Congress in 1847, but now he found himself exercising the same executive privileges that James K. Polk had employed during the Mexican War. Inevitably, the criticism against him was considerable.
Two days after the federal mobilization, the Union suffered a key blow when Virginia voted to secede. Mere months before, the Old Dominion had spearheaded a peace conference; now it was lost to the Union cause, shortly to become the heart of the insurgency. Hoping to counteract this defection, Lincoln promptly offered command of the Union armies to Virginian Robert E. Lee, who perforce declined and resigned his military position.
Within the next month, three more states would secede: Arkansas on May 6, Tennessee on May 7, and North Carolina, finding itself regionally isolated, on May 20. Then, on May 23, Virginia voted overwhelmingly to join the Confederacy, placing the rival factions face to face. Shortly thereafter, President Jefferson Davis pushed through a motion to shift the Confederate capital from Montgomery to Richmond, Virginia. As a result, the nerve centers of the Confederate and Union forces lay less than one hundred miles from each other. To be sure, the intervening valleys of Northern Virginia would become a crucial theater of war.