The Romulan Star Empire is one of the most influential powers in the Alpha Quadrant. It was founded about two thousand years ago by the Romulans, a civilization originated from planet Vulcan. When the Great Wars on Vulcan had ended and the great philosopher Surak tried to lead his people to a society that rejected emotions and was based on pure logic, some Vulcans totally opposed to the new pacifist philosophy tried to incite a revolt against Surak. They were banished from Vulcan and found a new home on two planets that later became known as Romulus and Remus. The Vulcan dissidents called themselves Romulans and founded the Romulan Star Empire.

In the following centuries, the Romulans pursued an aggressive expansion policy, leading to a war with the Vulcans that lasted for about one hundred years. In 2154, the Empire started a bitter war against the United Earth, following a botched first contact between the two powers. The war did not end until 2160, when Earth forces won a glorious victory over the Romulan fleet near Charon, resulting in the establishment of the Romulan Neutral Zone, a region of space with an average diameter of one light-year acting as a buffer between Earth and Romulan space. The peace treaty, which had been negotiated over subspace radio, provided that entry into the zone by either party would constitute an act of war. One result of the Earth-Romulan war was the foundation of the United Federation of Planets in 2161.

In 2266, after almost a century of isolation, the Romulans sent a sublight attack ship equipped with one of the first prototypes for a cloaking device into the Neutral Zone to test Federation defenses along the border. It was eventually destroyed by the starship USS Enterprise. In 2267, the Romulans were able to forge a tentative alliance with the Klingon Empire leading to an exchange of military technology like the cloaking device or the D7-type battle cruisers.

After the Tomed Incident in 2311, when a new war between the Federation and the Empire was barely prevented, the Romulans started another extended period of isolationism, during which there was no contact at all between the two powers. In the 2340s, the Romulans were involved in a bitter and violent border war with the Klingon Empire, also including the destruction of the Klingon outpost on Khitomer and Narendra III.

The Romulans did not emerge from isolation until 2364, when they sent a warbird to the Federation side of the Neutral Zone, blaiming Starfleet of having destroyed several outposts along the border, unbeknownst that this had been done by the Borg.

In 2369, Romulan dissidents yearning for a re-unification of Romulus and Vulcan arranged the defection of Vice-Proconsul M'ret to the Federation. Prior to this incident, the Romulan government tried to make use of the dissident movement to conceal their plans to invade Vulcan, an attempt that eventually failed thanks to Starfleet undercover operatives.

Although the Romulans considered the Dominion to be the greatest threat to the Alpha Quadrant in the last century, they agreed to a non-aggression treaty when the power from the Delta Quadrant annexed the Cardassian Union in 2372. The did not join forces with the Federation and the Klingon Empire until 2373, when Dominion plans to invade the Romulan Star Empire where discovered.

The formal leader of the Romulan Star Empire is the Empress, although the government is led by the Praetor (presently Praetor Neral). The Praetor is also chairman of the Romulan Continuing Committee, the executive body of the Empire. The legislative body is the Romulan Senate, meeting on Romulus and led by the Proconsul. This office was held by Neral prior to his appointment to Praetor in 2374. The imperial intelligence service is the Tal Shiar, a secret, often brutal, sometimes extragovernmental agency that enforces loyalty among the Romulan citizenry and military. Currently led by Koval.