Human-Romulan history
Human-Romulan history was mostly a series of confrontations between United Earth, and later the United Federation of Planets, and the Romulan Star Empire that once brought the two powers to an all out war. This was sometimes referred to as a game of chess and brinkmanship. ()

Return from Isolation 
After 53 years of isolation following the Tomed Incident, which had been resolved by the Treaty of Algeron, a buildup of seven Romulan battle cruisers were detected on approach to a Federation Neutral Zone outpost. The outpost requested reinforcements and the  and  responded. The arrival of the two Federation starships was a critical show of force that deterred the Romulans from taking any offensive action. ()

Later that year, several Federation outposts along the Romulan Neutral Zone suddenly disappeared. Starfleet ordered the Enterprise-D to investigate:

The Enterprise encountered a Romulan Warbird that had been investigating the disappearance of their own outposts along the Neutral Zone. After a tense standoff, both crews were able to determine that neither side was responsible for the attacks against the other. Nevertheless, the Romulans were unhappy with what they considered rapid expansion of the Federation while they remained in isolation during the last few decades. They promised they would no longer be "negligent" in this affair. ()

Search for the Iconian Gateway 
When Captain Donald Varley of the  received word of a possible Iconian gateway in the Neutral Zone, he felt it necessary to risk war by taking the Yamato into the Zone. The gateway would have allowed the Romulans to move legions of troops across star systems without the use of starships or conventional transporters. Varley felt it could not be allowed to fall into Romulan hands at any cost. The Yamato was destroyed when an Iconian computer virus caused a malfunction of the ship's antimatter containment system. As the Enterprise continued the investigation, it was confronted by the Romulan Warbird Haakona. The Haakona ordered the Enterprise to withdraw, but the Enterprise refused, stating they would not withdraw until they had determined the cause of the destruction of the Yamato. The Enterprise later located the Iconian gateway deeper within the Neutral Zone and destroyed it. ()

Galorndon Core Incident 
A Romulan scout ship crashed on the Federation planet Galorndon Core near the Romulan Neutral Zone. The USS Enterprise-D found one survivor and brought him on board. However the Federation vessel was ill-equipped to treat the gravely wounded Romulan soldier. A Romulan Warbird raced across the Neutral Zone to rescue the injured Romulan, however they were warned by the Enterprise that any Romulan ship that entered Federation space would be fired upon. Therefore, the Romulans demanded that the Enterprise rendezvous at the border so they can treat their injured soldier. The Enterprise was unwilling to leave as it had one of its own crewman trapped on the planet due to intense storms interfering with transporter activity. As a result, the Romulan later died of his injuries on the Enterprise. The Romulan commander was outraged and threatened to destroy the Enterprise in retaliation. Both vessels diverted powered to their weapons systems and locked on to each other. However, the Enterprise detected another Romulan survivor during a break in one of the storms. Captain Jean-Luc Picard offered to lower his shields to beam the Romulan survivor and the trapped Enterprise crewman on board. He reminded the Romulan that if the Enterprise was destroyed, so too would the peace between their two peoples. In the end, the Romulan commander did not fire and the second Romulan survivor was returned to his custody. ()

Nelvana III Incident 
A Romulan scout ship raced across the Neutral Zone while under close pursuit by a Romulan Warbird. When confronted by the USS Enterprise-D, the Warbird withdrew. The scout ship carried a Romulan soldier claiming to be a defector. He warned of an imminent Romulan attack from a hidden military base on Nelvana III inside the Neutral Zone. He claimed that the base would be operation within 48 hours and a fleet of Romulan Warbirds would be in striking range of fifteen Federation sectors. Starfleet Command ordered the  and  to head towards the Enterprises location, but they would not arrive before the 48 hour deadline. Captain Picard also requested reinforcements from the Klingon Defense Force. The Enterprise decided to launch a preemptive strike and destroy the base before it became operational. However, there was no base at Nelvana III. When the Enterprise arrived, two Romulan Warbirds had been lying in wait and ambushed the vessel. The Romulan commander demanded the Enterprises surrender. Captain Picard refused and signaled three Klingon warships to reveal themselves. The Klingon ships, which had followed the Enterprise under cloak, disengaged their cloaking devices and surrounded the Romulan ships. The Romulan commander decided to withdraw, narrowly averting a war once again. ()

Ambassador T'Pel incident 
The USS Enterprise-D was ordered to take Federation Ambassador  to the Romulan Neutral Zone to discuss the establishment of full diplomatic relations. However, as the ambassador attempted to beam aboard a Romulan Warbird to begin discussions, a transporter malfunction apparently killed her. Both ships returned to their respective territories. An investigation later determined that the accident had been faked and T'Pel was still on board the Romulan ship. The Enterprise immediately raced back to the Neutral Zone and confronted the Romulan ship. They armed their weapons systems and demanded that the Romulans return Ambassador T'Pel at once. Another Warbird decloaked, but the Enterprise refused to leave without the ambassador. The Romulan commander stated that he was not prepared to start a war that day and revealed to the Enterprise that T'Pel was in fact a Romulan intelligence operative. Since it was apparent that the Romulans were no longer holding a Federation citizen hostage, the Enterprise returned to Federation space without further incident. ()

Krios incident 
In 2367, the Romulans attempted to destroy the Federation-Klingon alliance by arming pro-independence rebels on the Klingon-held planet Krios Prime with fabricated Starfleet phaser rifles. In addition, Romulan agents kidnapped and brainwashed Lieutenant Commander Geordi La Forge into attempting to assassinate Governor Vagh. The Romulans hoped, unsuccessfully, to incite the Klingons into going to war by framing the Federation. ()

Klingon Civil War 
In 2368, the Romulans increased their clandestine efforts to influence Klingon politics, and thereby break the Federation-Klingon alliance, by directly supporting the House of Duras in the Klingon Civil War. Suspecting Romulan involvement, the Federation established a twenty-three ship blockade along the Klingon-Romulan border using a tachyon detection grid. The Federation obtained proof of Romulan involvement when several cloaked warbirds attempted to evade and cross the blockade. Having been discovered, the Romulans quickly withdrew their support from the Duras, ending the war. ()

Reunification attempt 
Several stolen Vulcan ships were used by the Romulan military to transport ground troops in an attempted invasion of Vulcan. They posed as members of Ambassador Spock's Vulcan-Romulan reunification movement. Spock himself discovered the plan and warned Federation authorities. Vulcan defense forces were mobilized and the USS Enterprise-D intercepted the invasion force in the Neutral Zone. The Romulans, unwilling to let their ground forces be captured by Starfleet, destroyed their own invasion force, killing two thousand of their own soldiers. () Despite the Romulan military's attempt to usurp their movement, the Romulan underground nevertheless continued to advocate for reunification and facilitated the defection of Vice-proconsul M'ret to the Federation. (; )

Development of interphase and phasing cloaking technology 
In 2368, the USS Enterprise-D responded to a distress call sent by a Romulan science ship with an apparently damaged graviton generator and a warp core breach in progress. In reality, the Romulan ship had been conducting secret experiments to develop an interphase cloaking device using a molecular phase inverter. To eliminate all possible Federation knowledge of the experiments, after accepting help from the Enterprise to resolve the core breach, the Romulan crew planned to surreptitiously generate a muon wave directed at the Enterprise dilithium crystal chamber, which would cause a catastrophic explosion the moment the Enterprise engaged warp drive. This threat was discovered and eliminated by Lieutenant Commander Geordi La Forge and Ensign Ro Laren. ()

In 2370, both Starfleet and the Romulans attempted to find and salvage the derelict , which was previously thought to have been destroyed in 2358. The intense renewed interest in the ship was due to the fact that the Pegasus carried a prototype Federation phasing cloaking device, developed in violation of the Treaty of Algeron, by which the Federation had pledged not to develop cloaking technology. Existence of the phasing cloak was acknowledged by the Federation to the Romulans when the device was briefly installed and used on the USS Enterprise-D. According to Captain Jean-Luc Picard, the scandal would likely result in courts martial prosecutions against senior officers of Starfleet Intelligence. ()

Devron system anomaly 
In one alternate timeline, a disruption of anti-time created a temporal anomaly in the Devron system, located in the Neutral Zone. Thirty Romulan Warbirds were sent to the border as well as fifteen Federation starships, including the USS Enterprise-D, the , and the . ()

USS Prometheus incident 
The Romulans learned of the top secret prototype  and sent troops to capture the vessel. The Starfleet crew was killed and the Romulans took the ship on a heading for Romulan territory. The  intercepted the Prometheus but was quickly disable in an exchange of fire. Three additional Federation starships intercepted the Prometheus in the Romulan Neutral Zone, where they were engaged by three Romulan Warbirds. The EMH mark II program on the Prometheus with the assistance of another fellow EMH managed to retake control of his ship and destroyed one of the Romulan Warbirds. The remaining two Romulan ships withdrew shortly thereafter. ()

Uniting against the Dominion 
In recognition of the need to jointly gather intelligence in the face of a threatened Dominion invasion of the Alpha Quadrant, in 2371 the Romulans consented to installing a cloaking device aboard the . () Although later that year the Romulan Tal Shiar and the Cardassian Obsidian Order jointly launched an unsactioned, and ultimately disastrous, preemptive strike against the Dominion, the Romulan government officially remained neutral in the Dominion War until 2374. () Unknown to the Romulans, the Federation successfully framed the Dominion for the assassination of Senator Vreenak, thereby causing the Romulans to enter the war on the side of the Federation and the Klingons. () Relations during the war improve to the point where a joint Federation-Romulan medical conference was convened on Romulus, although below the surface the Federation's secret Section 31 worked to turn Tal Shiar chairman Koval into a double agent. ()

Battle of the Bassen Rift 
Section 31 had predicted that at the end of the Dominion War, the only two remaining powers that could vie for control of the Alpha Quadrant were the Federation and the Romulans. (). Once the war had ended, the Romulan military pushed for an invasion but the Romulan Senate refused. The new Reman leader Shinzon pledged to destroy the Federation should he become the next Praetor. With the assistance of the Romulan military, the Senate was overthrown in a coup d'etat, and Shinzon was made the new Praetor of the Romulan Star Empire. Shinzon was in command of the Reman Warbird Scimitar – a heavily armed warship that had both conventional and biogenic weapons. The  was to lure the Scimitar to sector 1045 where a Federation fleet was lying in wait to attack the Scimitar . Instead the Scimitar chose to ambush the Enterprise first, inside the Bassen Rift where communications with Starfleet was not possible due to interference. After an exchange of fire between the Enterprise and the Scimitar, the Romulan military decided they would no longer support Shinzon. The military was only interest in the conquest of Earth, not mass genocide through the use of biogenic weapons. Two Romulan Warbirds arrived to assist the Enterprise. Both the Enterprise and the two Romulan ships were badly damaged in the ensuing battle, however the Scimitar was destroyed in the end. ()

Destruction of Romulus 
The cold war reached an eventual end in 2387 with the destruction of Romulus by a supernova, despite Spock's attempt to prevent the catastrophe. ()
History