In the days following the Dominion War, the Lazon II prison camp was liberated by Klingon forces. Riker and several other inmates boarded a Cardassian shuttle and left the planet.
He was believed to have begun gathering Maquis and other former DMZ colonists.
In early 2376 the United Maquis Confederacy declared their independence from both the Federation and Cardassian Union, signing treaties with the Klingon Empire, Ferengi Alliance, and the Bajoran Republic. Thomas Riker was in command of their ragtag fleet.
When the USS Lionheart entred Maquis space to provide reconstruction and security aid, Riker hijacked the ship and left the majority of its crew stranded on Aschelan V. After showing signs of mental instability, and beating Seff O'Rourke for the command codes, then resorting to a pretend rape of medic Gayle Hendricks, he tossed both of them in an escape pod and ejected it. ("Patriots")
His actions would be condemned publicly by the Keltic Confederation, but they did not remove him from their fledgling military.
The Maquis would later modify the Lionheart, rename it the UMS Patriot and make it their flagship.