After earning a fortune in the iron, coal, and steel industries by 1885, he retired from business to devote time to historical research.

That caused problems for his sister, who was finally allowed to marry the up-and-coming Republican businessman-politician Mark Hanna.

In the 1880s he was a Bourbon Democrat who supported Grover Cleveland and favored low tariffs, despite his own connection with the iron and steel industry.

His major work, History of the United States from the Compromise of 1850, was published in seven volumes, 1893–1906; the eight-volume edition appeared in 1920.

Working from newspaper accounts and published memoirs, Rhodes tracked the process by which major national decisions were made.

Unlike the first generation of historians, who had been personally deeply committed on the slavery issue, Rhodes approached it dispassionately.

The issue, he argued, was that the South fought to extend slavery – an institution condemned by ethics, Christianity, and the modern world.

[5] Rhodes treated slavery as a calamity for the South, but not a personal issue for white Southerners, who he thought deserved sympathy rather than censure.

Rhodes downplayed the importance of the abolitionist movement, instead focusing on mainstream leaders such as Daniel Webster for his promoting a deeper nationalism.

Lynch argued that, "The failure of the Reconstruction legislation was not due so much to the change of sentiment in the North as an unwise interpretation of these laws.

"[9] Rhodes has been described as one of the major scholars who gave "authoritative support to the Southern interpretation of history...[and] assumed the inferiority of the Negro...Rhodes, for example, in Volume VI of his monumental History of the United States, speaks of the freedmen as being 'Three and a half million of one of the most inferior races of mankind.