Romulans

The first Romulans were Vulcan dissidents who rebelled against the pacifist philosophy of Surak and settled on the twin planets, Romulus and Remus.  From there, the Romulan Star Empire spread across much of the quadrant with outposts on far-reaching worlds as Barradas 3, Calder 2, Dessica 2, and Yodalla Prime.  In 2160, the Neutral Zone was established after a four-year war between the Romulans and Earth forces, during which both sides used atomic weapons.  The zone was first violated by the Romulan incursion of 2266, and then in 2267 and 2268 by the original Starship Enterprise--the first time to expedite a cure for a hyper-aging disease and the second time to steal an advanced Romulan cloaking device.

The Romulan Star Empire formed an alliance with the Klingon Empire from 2268-2292, which allowed the Romulans to provide cloaking technology in exchange for Klingon spacecraft design.

After signing the Treaty of Algeron following the Tomed Incident in 2311, the Romulans had no contact with the Federation until 2364.  The mysterious destruction of Romulan and Federation outposts ended this isolationism when a Romulan warbird crossed the Neutral Zone to investigate.  Although the Borg were eventually revealed as the culprit behind those attacks, the incident triggered a resumption of hostilities between the Romulan Star Empire and the United Federation of Planets.

During that isolationist period, the Romulans pursued a policy of trying to covertly destabilize the Klingon government.  In 2367, the Romulan operative Sela kidnapped and mentally conditioned Starfleed officer Geordi La Forge in an attempt to assassinate Klingon governor Vagh and incite conflict between the Federation and the Klingons.  Sela subsequently formed a covert alliance with the House of Duras to wrest control of the Klingon High Council from Gowron.  Though their attempt was unsuccessful, it triggered a Klingon civil war in early 2368.  Later that year, the Romulan government attempted to use an underground reunification movement as a cover for a planetary invasion of Vulcan.