While serving as a colonel in First U.S. Cavalry, Sumner negotiated a prisoner release after the Battle of Black Jack as commanding officer of Fort Leavenworth. Subsequently, he dispersed the Free State legislature in Topeka, Kansas. In 1861, Sumner was called to serve as commander of the II Corps of the Army of the Potomac in his new rank of major general. He led this command through the Peninsula campaign of 1862, although Sumner was wounded twice during the campaign, He subsequently served at the Battle of Seven Pines, and later, at Antietam. At Fredericksburg, Sumner commanded the "Left Grand Division" of Burnside's army. He requested to resign upon the succession of Joseph Hooker as commander of the Army of the Potomac in January 1863.