A veteran of the Mexican War, and a major general in the state militia prior to the outbreak of hostilities, Burnside raised a regiment as a colonel and marched to war in 1861. He received his promotion to Brigadier General in August 1861, and to Major General in March 1862, as commander of the 9th Corps of the Army of the Potomac, in which capacity he served at Antietam. In November 1862, he replaced George B. McClellan as commander of the Army of the Potomac. His decisions in December 1862 at the Battle of Fredericksburg led to his replacement as commander of the army. In March 1863, he was assigned to the Department of the Ohio. In 1864, his old 9th corps was recruited to support the Army of the Potomac in the Overland Campaign of 1864. However, his performance at the Siege of Petersburg led his resignation in April 1865 from service.