James Garfield achieved the rank of major general by age 32

According to James Garfield's facts, he participated significantly in the Civil War despite having no prior combat training. He joined the newly formed Republican Party as a proponent of free-soil ideas (against the expansion of slavery) and was elected to the Ohio assembly in 1859. He assisted in the 42nd Ohio Volunteer Infantry's recruitment during the Civil War, rising to the rank of colonel. He was elected to the U.S. House of Representatives in April 1862 after leading a brigade at the Battle of Shiloh. While waiting for Congress to convene, he served as the Army of the Cumberland's chief of staff, earning promotion to major general after excelling at the Battle of Chickamauga (September 1863).

Garfield served as the 19th congressional district's representative for Ohio for nine terms until 1880. He developed his expertise in financial problems while serving as the chairman of the House Committee on Appropriations. He promoted a high protective tariff and, as a Radical Republican, pursued a solid policy of Reconstruction for the South. The Ohio legislature chose him to serve in the Senate in 1880.