Galion was colonized at some point prior to the mid-24th century, with numerous cities populated by Humans and other Federation citizens. When the Federation-Cardassian Treaty of the year 2370 ceded numerous worlds between the Federation and the Cardassian Union, Galion ended up being part of the Demilitarized Zone, and was a haven for Maquis leadership fighting Cardassian abuses of the terms of the treaty.
Ro Laren ended up building a life on Galion after her Maquis activities made her a fugitive from the Federation. She was living with a romantic partner, Derek, in 2373 when the Dominion War broke out.
A number of Maquis admirals were in attendance of the Maquis community on Galion in 2374, When the civilian leadership of the Galion colonists petitioned the Dominion to recognize their claim to the world, the Dominion made an official refusal, and ordered the inhabitants to evacuate. Various spacecraft were able to leave the world, but few had a chance of escaping the Jem'Hadar. With over half-a-million people still residing on the world, in violation of the Dominion ultimatum, the Dominion ordered the complete eradication of the planet's population. The Maquis leadership escaped on a freighter flying under a false flag, masquerading as a neutral Bajoran transport starship. (TNG - The Dominion War novel: Behind Enemy Lines)