Originally Posted by kevin
They weren't known at all til TNG retconned an entire 20 year war with them, who knows when they made first contact!!!
While it's true that there was never any previous mention of the Cardassians until their first appearance in TNG's "The Wounded," the same was true of the Klingons in TOS' "Errand of Mercy" and they were later made out to be one of the Federation's oldest enemies afterwards.
Subsequent TNG episodes gave a bit more background on the Cardassians, including that Spock and Sarek had a major difference of opinion over the Federation's initial dealing with them possibly sometime in the mid to early 24th-Century...