Wikipedia
Sixth Column, also known under the title The Day After Tomorrow, is a science fiction novel by Robert A. Heinlein, based on a story by editor John W. Campbell, and set in a United States that has been conquered by the PanAsians, a combination of Chinese and Japanese. Originally published as a serial in Astounding Science Fiction (January, February, March 1941, using the pen name Anson MacDonald) it was published in hardcover in 1949.
Usage examples of "sixth column".
He knew now that he had been right when he had decided on a sixth column.
So I wrote MAGIC, INCORPORATED and started east on the proceeds, and wrote THEY and SIXTH COLUMN while I was on that trip.
Heinlein, who wrote a much longer novel based on it (Sixth Column, currently known as The Day After Tomorrow, Signet).
Now, with the sixth column ready to be moved, he saw to it that the iron rings were firmly attached to hold the massive weight, making it possible to lessen the bulk of rope.
They will be our sixth column, our secret army, our private traitors at the heart of your government, sowing chaos and confusion as we require.
Not even the most minute notice at the bottom of the sixth column on the fourth page.