Eloh is a subject world of the Romulan Star Empire, located near the Romulan Neutral Zone as well as the borders with the Klingon Empire and United Federation of Planets. It is the homeworld of the Elohsians. (TNG novel: The Romulan Stratagem)
The Elohsian people were at war with one another for centuries, finally establishing a unitary, planet-wide government in 2368. Much of the planet had suffered damage and there was still great suspicion among the various former parties.
In 2369, the Elohsians achieved warp technology and made contact with the Empire and with the Federation, requesting that both governments send ships to meet with the Elohsian government in advance of a planet-wide referendum to choose which organization to join. The Federation dispatched the USS Enterprise-D under Captain Jean-Luc Picard and the Romulans dispatched a Warbird under Commander Sela.
A week of diplomatic engagements preceded the vote in an attempt to introduce the Elohsian people to the two empires. A terrorist attack was initially blamed on a Starfleet engineer, Lieutenant Commander Geordi La Forge. Eventually it was revealed that the attack had been a Tal Shiar plan to push the planet into the Romulan camp. Regardless of this information, the Elohsians opted to join the Romulan Empire, in appreciation for the unifying powers of its strict rule. (TNG novel: The Romulan Stratagem)