An Ohio native, "Old Rosy" graduated from West Point in 1842 and later became a professor there. It was during this period that he converted to Catholicism (he was raised Methodist). His explanation to his family prompted the conversion of his younger brother, Sylvester, who would become the first bishop of Columbus. In 1854, he went into business, but volunteered to serve at the outbreak of the Civil War. He rose to the rank of commander of the Army of the Mississippi in the west and then, commander of the Army of the Cumberland as major general. He was relieved of his command after the Battle of Chickamauga in 1863 and after the war worked in railroads and Democratic politics.