Garfield later served under Major General Don Carlos Buell at the Battle of Shiloh and under Thomas J. Wood at the siege of Corinth. Garfield's health deteriorated and he was sent to serve on a commission to investigate the conduct of Union general Fitz John Porter. In the spring of 1863, Garfield returned to the field as chief of staff for William S. Rosecrans, commander of the Army of the Cumberland. After the disastrous Chickamauga campaign in September 1863, Rosecrans was relieved of his command. Garfield saved his own reputation by fighting bravely during the battles, and he was subsequently promoted to the rank of major general. Garfield's fame spread, and William Dennison engineered Garfield's 1863 election to Congress. As Whitelaw Reid commented, Garfield was "the most able and prominent of the young politicians who entered the army at the outbreak of the war." Garfield did not want to leave the army, so he personally visited President Abraham Lincoln for advice on the matter. Lincoln told Garfield that he had more generals than he could handle, and what he needed was political support.