Derna was the small and uninhabited fourth moon of Bajor.
In early 2375, Senator Cretak of the Romulan Star Empire asked Colonel Kira Nerys if her people could place a field hospital on the moon as large numbers of Romulans were dying on the long trip back to Romulus. The Bajoran Provisional Government quickly accepted this request from their allies and the hospital was constructed.
However, when a Starfleet transport carrying mainly injured Vulcans was denied access to the hospital, evidence emerged that the Romulans had begun arming the facility with plasma torpedoes. The Bajoran government objected to this action and demanded that the Romulans remove the weapons. The Romulans refused, forcing the two allies into a stand-off.
Colonel Kira took a small fleet of Bajoran impulse ships to prevent an approaching group of fourteen warbirds from reaching the moon. The ships, clearly no match even for a single warbird, held their ground until Starfleet Admiral William Ross told Cretak that if she did not remove the weapons, he would. (DS9: "Image in the Sand", "Shadows and Symbols")