In 2374 the Romulan Star Empire entered the Dominion War on the side of the Federation, after a plot by Sisko and Garak to convince it of a Dominion threat.
In 2374 the Federation and the Klingons were losing the war, while the Romulans kept their nonaggression pact with the Dominion. Sisko turned to Garak to obtain proof that the Dominion planned to attack Romulus. When none could be found, Garak arranged a forged holographic recording of Dominion leaders discussing an invasion.
Sisko showed the recording to Senator Vreenak, who visited Deep Space Nine in secret and soon saw that it was a fake. On his way home, Vreenak's shuttle was destroyed by a bomb planted by Garak. The Romulans examined the wreckage, believed the damaged recording was real and concluded the Dominion had killed Vreenak to hide its plans.
The Romulan Empire declared war on the Dominion, giving the Federation and the Klingons a powerful ally. Sisko came to terms with the lies and deaths behind it, deciding that the result was worth the cost to his conscience.