Sixth Column, alternatively titled The Day After Tomorrow, is a short novel by Robert A. Heinlein. Originally published as a serial in Astounding Science Fiction (January, February, March 1941 under the pen name of Anson MacDonald, it was published in hardcover in 1949. The novel follows the tried and true Yellow Peril storyline of an invasion/occupation of North America by East Asians, and in this case by the PanAsians, an absurd fusion of the Chinese and Japanese.
A top secret research facility hidden in the Colorado mountains is the last remaining outpost of the United States Army after its defeat by the PanAsians. The conquerors had absorbed the Soviets after being attacked by them and had then gone on to absorb India as well. In keeping with the tradition of anti-East Asian Yellow Peril fiction, the invaders are depicted as ruthless and cruel. Noting that the invaders have allowed the free practice of religion (the better to pacify their slaves), the Americans set up a church of their own in order to build a resistance movement—the Sixth Column (as opposed to a treasonous fifth column).
The secret lab is in turmoil as the novel begins. All but six of the personnel have died suddenly, due to unknown forces released by an experiment operating within the newly discovered magneto-gravitic or electro-gravitic spectra. The surviving scientists soon learn that they can selectively kill people by releasing the internal pressure of their cell membranes, among other things. Using this discovery, they construct a race-selective weapon that will kill only Asians.