Valan III was a planet in the Alpha Quadrant, the site of a human colony in the mid 24th century. It was turned over by the United Federation of Planets to the Cardassian Union in 2370, and placed in the Demilitarized Zone created by treaty signed that year. The Cardassian rulers severely curtailed civil liberties on Valan III, and later sabotaged the colony's replicators, leading to several deaths, including a girl named Taliana, the daughter of future Maquis leader Maass.
Valan III was the home base of Calvin Hudson, the Starfleet liaison to the colony at to the ruling Cardassians. In part because of this, Valan III was considered the birthplace of the Maquis. (TNG - Slings and Arrows eBook: A Weary Life)