Klingon propaganda in the 23rd century claimed that the United Federation of Planets committed atrocities against its prisoners, placing them in death camps and torturing them for their scientific and military secrets. When beamed aboard the USS Enterprise in 2268, Mara reminded her husband Kang of these claims. Captain James T. Kirk remarked that she had much to learn about the Federation.
During the same incident, Leonard McCoy's own paranoia left him to believe that Klingons treated their prisoners to their to atrocities as slave labor, death planets, experiments. (TOS: "Day of the Dove")