Star Trek: Deep Space Nine, In the Pale Moonlight. First aired April 15, 1998.
Set on a distant Federation outpost jammed with a diverse mixture of species, Deep Space Nine began as a confused religio-cultural drama and ended, much improved, as a tense series about a long war between the Federation and a powerful enemy known as the Dominion. This was the best of the war episodes: The Federation is losing; friends are dying; the planet Betazed (home world of Enterprise's Counselor Deanna Troi) has fallen. Captain Sisko hatches a complicated plan to fabricate evidence showing that the Dominion wants to conquer the Romulans. His aim is to bring the Romulans into the war on the Federation's side. As Sisko gives up his principles slowly, one by one, in order to make his plan work, you expect Trek's simple moral verities to prevail. It is dumbfounding, and chilling, when they don't.
Next Star Trek (2009)