Star Trek: Deep Space Nine

In the Pale Moonlight (6x19)

Air date: Apr 15, 1998

Stardate: 51721.3 Sisko and Garak lure the Romulans into the Dominion war in order to get them to join the Federation/Klingon Alliance. Without them, the Federation and Klingon Empire would eventually lose the war and fall under Dominion rule.