The Battle of Psellus occurred in late 2371 between the United Federation of Planets and the Romulan Star Empire when elements of the latter's government invaded the neutral planet of Pselus III moments before its leadership was to sign an armistice between its warring factions and become a Federation protectorate.
While the majority of the senior staff were either at the armistice signing or setting up a medical clinic on the planet, the USS Lionheart was in orbit under the command of its second officer, Lieutenant Commander Nikolas Stone. Just as he received reports of a firefight at the armistice, twenty-four Romulan Star Navy vessels decloaked throughout the Pesllus System. Stone managed to destroy two troop transports and cripple a frigate as he fought a retreat to rendezvous with a Starfleet Task Force led by the USS Himori.
On the planet, a derelict Romulan Bird of Prey is found deep in the swamps. Along with the Lionheart's medical technician, Meridian is able to restore partial power, and fires a volley of three plasma torpedoes at vessels into orbit, destroying a warbird. He and the Psellian guerillas escape with two torpedo warheads, sidearms and rations before an orbital volley destroys the vessel. ("Armistice Part I")
As Romulan and Starfleet forces marshaled around the system, Commander Kiva, Lieutenant Christoffer Williams, Envoy Cal West, Lieutenant Junior Grade Graw Arkan, and Ensign Spencer Stone were being held as prisoners in the Psellian capital. Arrangements are being made to transfer them to Romulus.
Meanwhile, Lieutenant Junior Grade Meridian Janos, a former member of the Bajoran Resistance and a Starfleet's Rapid Response Teams, along with the Lionheart's medical staff and Psellian citizens, began waging a guerrilla war against the Romulans from the planet's swamps.
At the system's edge, the Lionheart escorted the USS Oracle to the system's edge to scan the area with a new system based on Vulcan scientist Sarok's design for detecting cloaked vessels. Three Romulan fighters moved to intercept the pair.
("Armistice Part II") While the battle was waged other incidents were going on along the Neutral Zone.