Tahna Los fought with Kira Nerys in the Bajoran Resistance during the Cardassian occupation of Bajor. However, after the occupation ended, Tahna joined a terrorist group called the Kohn-Ma. The group was dedicated to continuing to punish the Cardassians, and also those Bajorans who disagreed with their views. They were opposed to the Bajoran provisional government's decision to request assistance from the Federation, believing Bajor should stand alone.
Tahna stole an anti-matter generator from the Cardassians, then fled back to Bajoran space with the Cardassians in pursuit. Although his ship was destroyed, he was safely beamed aboard Deep Space 9. He requested asylum, which Sisko somewhat reluctantly granted. Reunited with Kira, he told her that he wanted to give up violence and return to Bajor, and that others of the Kohn-Ma wanted the same. Kira agreed to help him.
However, Tahna had already arranged to meet the Duras sisters Lursa and B'Etor on the station: They had agreed to sell him a canister of bilitrium, which he could use with the anti-matter generator to create a powerful bomb. The other Kohn-Ma members were bringing in the payment. Tahna admitted to Kira that he had known she was on the station and was still with the Kohn-Ma but also told her he had a non-violent way to end Bajor's problems and needed her to provide him with a ship.
Kira met Tahna with a Runabout and they rendezvoused with the Duras sisters in space, with Tahna receiving the components and assembling the bomb. It was at that point that he realised both Starfleet (who had uncovered his plan) and the Cardassians (who had been alerted by the Duras sisters) were on an intercept course. Tahna revealed to Kira that he was not planning to destroy Deep Space 9, as had been assumed, but to collapse the mouth of the wormhole: His thinking was that, with it gone, Bajor would cease to be of interest to both the Federation and the Cardassians and be free of alien influences.
Kira, who was still working with Starfleet, fought with Tahna and delayed him launching the bomb. Their Runabout tumbled through the wormhole and the bomb was launched in the Gamma Quadrant, exploding harmlessly. When the Runabout Ganges arrived, Tahna attempted to use Kira as a hostage but Sisko calmly pointed out that he could either surrender to him or wait for the Cardassians to catch up: Tahna chose the former. As he was taken back to Deep Space 9 and placed in custody, Kira tried to convince him that the old ways didn't work anymore but Tahna considered her a traitor.