The Derna incident was an event that took place in 2375, during the Dominion War.
Shortly after the Romulan Star Empire assigned Senator Kimara Cretak and her staff to Deep Space 9, Cretak asked Colonel Kira Nerys to use her influence with the Bajoran government to give the Romulan military permission to establish a hospital on Derna, the fourth moon of Bajor. Later, Odo highlighted the fact that a Starfleet vessel with injured personnel was turned away from the hospital and that trilithium isotope readings indicated that plasma torpedoes have been installed at the site. Kira confronted Senator Cretak with her findings and warns her she will force the Romulans to remove the weapons, which were considered a threat to Bajor. Weyoun soon learned of the incident, and he and Damar hoped it could end the Federation Alliance. (DS9 episode: "Image in the Sand")
The Federation Council soon protested the situation to the Romulan Senate, but the two came to an impasse. Admiral William Ross was reluctant to involve Starfleet, due to the importance of the Romulans to the war effort. Colonel Kira soon decided to blockade the moon before the Romulans could bring in launch sequencers for the torpedoes. Unfortunately, the Bajoran Militia are only able to give Kira twelve impulse vessels, although Kira admitted she hoped the Romulans wouldn't fire on them, lest they risk the ire of Starfleet and the Federation. The Romulans soon arrived at Derna, comprising a fleet of fourteen D'deridex-class warbirds. Kira stood firm, and Admiral Ross intervened to end the incident, threatening Cretak with Starfleet involvement to remove the weapons. (DS9 episode: "Shadows and Symbols")