The Ennis and Nol-Ennis were sentenced to eternal warfare on the moon by their planet's leaders to set an example when they could not stop warring. Shel-la could not remember by 2369 what the original war was for. He thought it might have been over water or land.
Artificial microbes were deployed on the moon to prevent the Ennis and Nol-Ennis from permanently dying and avoiding the sentence. Shel-la told Benjamin Sisko, after the runabout Yangtzee Kiang crash landed on the moon, that he had died countless times.
The artificial microbes also had the effect of making anyone revived by them permanently dependent on them, and the microbes were designed to work only in the moon's ecosystem. This had the effect of preventing anyone who had ever been revived by the microbes from leaving the moon.
When Dr. Julian Bashir thought he should perhaps work on reprogramming the microbes to get them not revive the moon's inhabitants, thus ending their torture, Shel-la's first thought was that Bashir's work could be used as the ultimate weapon against the Nol-Ennis, wiping them out once and for all.