Unsatisfied with the fate of the Cardassian Union, Kentar began a massive build up of military forces which struck the Federation during their incursion into the Delta Quadrant via a transwarp portal to combat the Borg Collective. This endeavour made Starfleet vulnerable and would have resorted in their fleet being decimated by the Cardassian fleet if not for the protection of the Klingon Empire. While Federation was busy fighting the Borg, the Klingons began a war to stop the Union's war capability.
Chancellor Martok eventually discovered that the Romulans were supplying Kentar, and led an attack against the sector that the Romulans were using as their base. During the battle, Kentar led a personal assault on Martok forces, only to be beaten back.
The rebellion collapsed when Cardassia was invaded. In an attempt to crush the rebellion once and for all, Chancellor Martok led an assault on Cardassia Prime. Kentar did his best to defend the system, but Martok's forces over-ran the Cardassians, and Kentar fled again.
Kentar's stronghold in the Hurik system was invaded afterward. The Hurik base was where the Cardassians were experimenting with technology on how to bring Species 8472 into the galaxy. Once again, Martok led the Klingon assault. Kentar's forces were once again defeated, with Kentar killed and his ship being destroyed in the battle. (TNG video game: Armada II)