Maquis

The Maquis was a paramilitary group between 2370 and 2373. The Maquis consisted primarily of former citizens of the United Federation of Planets whose home worlds were ceded to the Cardassian Union by the Treaty of 2370. Having to live under Cardassian domain without Federation support, the Maquis (named after a similar anti-Nazi resistance group during Earth's World War II) used violent means to counter Cardassian oppression. (DS9 episode: "The Maquis, Part I")

After a disastrous strike on the Founder Homeworld, the Obsidian Order lost most of its personnel and equipment. As a result, the Detapa Council grew in strength, and the Cardassian state fell in to chaos. Also the Klingons chose to invade at about the same time. As a result, the Maquis had an almost totally free hand in the Demilitarized Zone, and were about to declare their independence.

All this changed when the Cardassian Union and the Dominion formed an alliance. The Maquis was almost totally obliterated by Dominion and Cardassian forces. (DS9 episode: "By Inferno's Light"). Only a few survivors remained, including those on USS Voyager. Aside from those on Voyager, a number of other former Maquis returned to Starfleet during the Dominion War. Those who fought for Starfleet in the Dominion War were granted pardons by the Federation, and this blanket pardon was later extended to the Maquis on Voyager after they returned to Earth. (VOY novelization of End Game).