Who are the Rihannsu? More commonly known at the Romulans. A race of people who were once fellow inhabitants of the planet Vulcan who left during the Surak Reforms. It is believed, as most species have, that the Romulans were moderates of Vulcan society. Neither the fully and extreme emotional destroyers nor those who felt the need to suppress their emotions. Feeling forced to assimilate a behavior they were opposed to, they fled Vulcan in search of a new home. This exodus took place during Earth's 3rd century and the Romulan calendar is based off of this exodus.
After finding a suitable home-world, naming it Romulus, they began to build and expand an empire. In the year 1952 (Terran year 2152), they first encountered the Terran's. An encounter that would lead to all out war. One that would find them allying with a long time enemy, the Klingons, in a technology exchange that would benefit both worlds and hopefully crush their common enemy. The technological advancements of the Terran's, with help from their old brethren the Vulcans, the war entered a stalemate. This stalemate would last for a hundred years by the Treaty of Algeron. A cold war. Contact eventually was broken and tensions rose after an encounter in 2164 (2364 Terran). Since that time, it had been a chess game with the Federation. A brief alliance was even formed with the rising threat of the Dominion.
After the disastrous attempt by the Tal Shiar to eliminate The Founders in the Gamma Quadrant, the Empire has been in disarray. Many colonies have seceded, some becoming fully independent, some defecting to other galactic powers. And with each secession, some of the power of the Empire is eroded as well as some of the fleet of the Romulan Star Navy.