E. C. Bearss, "Sumner, Edwin Vose," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00970.html.
In February 1861, as the southern states seceded, Sumner was one of three army officers that accompanied President-elect Abraham Lincoln on his train trip from Springfield, Illinois, to Washington, D.C. Upon the dismissal of David E. Twiggs from the service, Sumner was promoted to brigadier general on 16 March 1861, becoming one of three regular army brigadiers. One week later he was ordered to San Francisco to relieve Colonel Albert Sidney Johnston as commander of the Department of the Pacific. Traveling by way of Panama, Sumner held this post until 20 October, when he returned to the East Coast. Reporting to Major General George B. McClellan on 25 November, Sumner was assigned to command one of the Army of the Potomac's twelve infantry divisions. President Lincoln, becoming increasingly disenchanted with McClellan and his failure to boldly seize the initiative, issued orders in March 1862 for the Army of the Potomac to be organized into five corps. Sumner was given command of the Second, and he thereby became the oldest corps commander in the Army of the Potomac.