After Thomas betrayed the Federation to join the Maquis, he was captured by the Cardassians and sent to a prison camp. When the camp was abandoned, the prisoners where left to fend for themselves, Joshua was eventually made leader after his father died of heart failure. (Star Trek Online)
Encounter with the Constance
The group was held captive for a short time before in 2409, the USS Constance reached the planet as part of an investigation into True Way activity. While the rest of the Maquis where transported aboard the Constance, Joshua stayed behind to assist the away team in taking the base's commander, Gul Boraj.
Captain Rachel Covaks was in communication with Starfleet Command, attempting to negotiate on behalf of the Maquis. She offered to allow the Maquis that could to stay on board as members of her crew. (Star Trek: Constance)