The Romulan Civil War (also known as the Second Romulan War) was a conflict in 2276, between the Romulan Star Empire and the United Federation of Planets and the Free Rihannsu Movement, which rebelled against the authority of the Empire. Despite long odds, the Free Rihannsu defeated the Imperial Forces, due to the overstretching of the Grand Fleet, and the superior tactics of the Rebels led by Ael t'Rllaillieu and James T. Kirk. (TOS - Rihannsu novel: The Empty Chair)
Road to War
Throughout the 2260s decade, skirmishes had taken place, as a more involved Romulan Star Empire began to test the defenses of the Federation, first and most notably at the Battle of Icarus IV.Starfleet, interested in Romulan technology, launched a series of covert operations to steal Romulan hardware, most notably the cloaking device. (TOS episodes: "Balance of Terror", "The Enterprise Incident")
Events within the Star Empire itself also led to the war. The Romulans became dissatisfied with their treaty with the Klingons, which they believed was more beneficial to the Klingons. Also, corrupt Praetors within the Tricameron began the treacherous project such as the Vulcan neural project at Levaeri V, and Sunseed. While many in the Empire became nervous of the government, one noted commander took action: Ael t'Rllaillieu.
In 2275, Ael spoke out against the project on Levaeri V, fearing that the Praetors with Vulcan neural abilities would transform the Empire into a police state. She was "rewarded" by being given a ship destined to be destroyed in patrol action in the Romulan Neutral Zone. Ael however escaped onto her ship the ChR Bloodwing, and went to the only people she had left, her enemies of the Federation. She made contact with the USS Enterprise, and persuaded its captain James T. Kirk to help her, with the capture of the Vulcan crewed USS Intrepid, Kirk saw no choice but to assist. Fooling the Grand Fleet into thinking the Enterprise had been captured, the two ships made it to the base and destroyed it. (TOS novel: My Enemy, My Ally)
The next year, Bloodwing returned from exile to Romulus, working with Starfleet Intelligence to retrieve Doctor Leonard McCoy from custody. While in the Senate chamber, Ael took the sacred Sword of S'task and left with McCoy. (TOS novel: The Romulan Way)
Conference at RV Trianguli
With tensions between the Romulans and Federation at an all time high, diplomats from both sides met at RV Trianguli near the Neutral Zone, to discuss a diplomatic solution. The Romulans had different plans, the colony worlds began to revolt against the Empire, using Ael as a rallying cry. An ambush was launched at 15 Trianguli, to destroy Bloodwing and Enterprise. The attack was barely defeated, and both ships were lucky to escape. The conference itself did not go well either, as the Romulans delivered a four point ultimatum to the Federation delegation. Information received from Federation undercover agent Terise Haleakala-LoBrutto suggested the Romulans were planning a preemptive strike. The attack nearly destroyed the ships of the Federation delegation. Bloodwing entered space to aid in the Free Rihannsu revolt and Enterprise, following orders, followed her. Unknown to the Federation at the time, a cloaked shuttlecraft armed with the Sunseed weapon was launched at RV Trianguli on a course towards Sol. (TOS - Rihannsu novel: Honor Blade)
The two ships headed for the nearest rebelling planet at Artaleirh. Using "home field" advantage, the Enterprise and Bloodwing lured the attacking force of nine Grand Fleet vessels into and asteroid field, thus destroying or disabling the Romulan fleet. However, soon the force was met by a Klingon force hoping to destroy the tired survivors. The Enterprise and Bloodwing were saved by intervention of the Romulan Clan-Ship Tyrava.
Kirk was then placed in command of the Free Rihannsu Fleet and decided to push and meet the rest of the Grand Fleet at Augo. Despite Kirk's best planning, the battle turned chaotic and victory was only assured with the intervention of a Federation force.
On Romulus, things got even more desperate, as the government tried to prevent an uprising on the homeworld as well. Arrhae (Terise) was warned about the Sunseed probe and warned Enterprise.
Kirk meanwhile decided to push on to Romulus itself. Using Praetor Gurrhim to spread information of his "death", the public offered no resistance to the invasion. The fleet destroyed Grand Fleet headquarters, and landed on the planet after interdicting subspace communication and transporters. Kirk and Ael were met by sympathetic senators including Arrhae who joined the group into the city. The Three activated the Sunseed probe, but the attempt was defeated with the combined help of the engineers of both Enterprise and Bloodwing.
In the aftermath of the Romulan Civil War, Ael reluctantly agreed to become Empress in order to sow unity and regain honor for the Romulan people. The Empire was at peace in most of Ael's reign, and the Empire became quieter to its neighbours. The Empire would not become active again until the 2360s. (TNG episode: "The Neutral Zone")