Sixth Column is a 1949 science fiction novel by Robert Heinlein, set in a United States that has been conquered by a foreign invader. It centers on the activites of a research lab hidden in the Colorado mountains, which is is the last remaining outpost of the United States Army after its defeat by the Pan-Asians. The conquerors had absorbed the Soviets after an attack by them and had gone on to amalgamate India as well. The invaders are depicted as ruthless and cruel - for example, they crush an abortive rebellion by killing 150,000 American civilians as punishment.
The book is notable for its frank and controversial portrayal of racism. The Pan-Asian conquerors regard themselves as a chosen people predestined to rule over lesser races, and they refer to white people as slaves. "Three things only do slaves require: work, food, and their religion." They require outward signs of respect, such as jumping promptly into the gutter when a member of the chosen race walks by, and the slightest hesitation to show the prescribed courtesies earns a swagger stick across the face. One character is Frank Mitsui, a hunted man mourning the murder of his wife and children by the invaders because "they did not fit" in the new PanAsiatic racial order. The Americans in the novel respond to their conquerors's racism by often referring to them in unflattering terms, such as flat face, slanties (for the slanted eye folds typical of Asian genetics), and monkey boys.
Some people view both Sixth Column and Farnham's Freehold, another novel by Heinlein, as proof that Heinlein is a racist. A more accurate statement would be that Heinlein made strong philosophical points about racism in these two works, and that some people who disagree resort to attacking the author's reputation rather than answering his arguments.