When the American Civil War broke out, the Blackwell sisters aided in nursing efforts. Blackwell sympathized heavily with the North due to her abolitionist roots, and even went so far as to say she would have left the country if the North had compromised on the subject of slavery. However, Blackwell did meet with some resistance on the part of the male-dominated United States Sanitary Commission (USSC) . The male physicians refused to help with the nurse education plan if it involved the Blackwells. In response to the USSC, Blackwell organized with the Woman's Central Relief Association (WCRA). The WCRA worked against the problem of uncoordinated benevolence, but ultimately was absorbed by the USSC. Still, the New York Infirmary managed to work with Dorothea Dix to train nurses for the Union effort.
Major Robert Anderson, commander of the Union's Fort Sumter in Charleston, South Carolina, sent a request for provisions to Washington, and Lincoln's order to meet that request was seen by the secessionists as an act of war. On April 12, 1861, Confederate forces fired on Union troops at Fort Sumter and began the fight.
On April 15, Lincoln called on the states to send detachments totaling 75,000 troops to recapture forts, protect Washington, and "preserve the Union", which, in his view, remained intact despite the seceding states. This call forced states to choose sides. Virginia seceded and was rewarded with the designation of Richmond as the Confederate capital, despite its exposure to Union lines. North Carolina, Tennessee, and Arkansas followed over the following two months. Secession sentiment was strong in Missouri and Maryland, but did not prevail; Kentucky remained neutral. The Fort Sumter attack rallied Americans north of the Mason-Dixon line to defend the nation.
As States sent Union regiments south, on April 19, Baltimore mobs in control of the rail links attacked Union troops who were changing trains. Local leaders' groups later burned critical rail bridges to the capital and the Army responded by arresting local Maryland officials. Lincoln suspended the writ of habeas corpus where needed for the security of troops trying to reach Washington. John Merryman, one Maryland official hindering the U.S. troop movements, petitioned Supreme Court Chief Justice Roger B. Taney to issue a writ of habeas corpus. In June Taney, ruling only for the lower circuit court in ex parte Merryman, issued the writ which he felt could only be suspended by Congress. Lincoln persisted with the policy of suspension in select areas.