New Beijing was a Federation colony, once with a population of over five thousand.
During the Dominion War, the Dominion obtained intelligence suggesting that biogenic weaponry was being developed on New Beijing, and sent the Jem'Hadar to cleanse the planet. The massacre killed all but one inhabitant of the planet, Dr. Ethan Locken. The appalling tragedy allowed Section 31 to convince Locken to join their ranks.
Julian Bashir eventually theorized that the false intelligence was actually fabricated by Section 31 operative Cole, and that the agent also ensured that the planetary defenses were offline and that Locken was able to find a safe place to hide during the attack. No evidence has surfaced to confirm this, with the sole exception of the discovery by Dr. Bashir of Agent Cole's undercover identity, Locken's mentor Doctor Murdoch, who was also present on New Bejing during the massacre. (DS9 - Section 31 novel: Abyss)