Star Trek: Deep Space Nine: Season 6, Episode 19

6 - 19: In the Pale Moonlight

With mounting losses in the Federation-Dominion war, and the looming specter of defeat, Captain Sisko enlists Garak's help to bring the Romulan Empire into the war. However, Sisko may have to sell his soul and the values Starfleet stands for in order to achieve his mission.