The Antwerp Conference was a diplomatic meeting between the United Federation of Planets and the Romulan Star Empire that took place in the Belgian city of Antwerp in 2372. A Tholian was also present as an observer.
The conference was infiltrated by a Founder of the Dominion, who planted and detonated a bomb that killed the twenty-seven people in attendance. The bombing later spurred Admiral James Leyton into getting martial law declared on Earth to protect it. (DS9 episodes: "Homefront", "Paradise Lost")
Many key Starfleet personnel were killed in the bombing. There were concerns the Dominion would try to attack a conference on Deep Space 9 later that year and weaken Starfleet further. (DS9 novel: The Heart of the Warrior)
The Federation tried to restart the talks with the Romulans in hope of an alliance against the Dominion. They were unsuccessful and relations between the two powers became frosty again. (DS9 novel: The Badlands, Part IV)