The Unification movement consisted of Romulans who were against the militaristic nature of the empire and wished for a re-unification with their Vulcan cousins. The movement was a powerful force but one that was suppressed by the Romulan authorities.
The group was a charismatic force within Romulan society and was led by Ambassador Spock. It was nearly quashed by Commander Sela during the attempted invasion of Vulcan in 2368, but still operated as of 2371. It was one of the many factions that competed for power after the collapse of the Romulan Senate during Shinzon's coup. (TTN novels: , Taking Wing, TNG episode & novelization: Unification, TOS - Spock: Reflections comics: "Issue 1", "Issue 4")
In 2381, Spock returned to Federation space as the movement grew. Xiomek worked with the movement seeing the Remans and Unification members as allies, both exploited by the Romulan government. On stardate 59480.33, a formal aid request was made by Spock to the Federation Council and the matter was taken into consideration. (ST website: The Path to 2409)
2382 saw the movement petition Tal'aura to be represented (along with the Remans), but she ignored it. After this, the Spock brought their case to the Federation Council who formally took up the matter. The issue was tabled after Councilor T'Los influenced the council with logic stating the separation would likely continue and in order to "protect the Vulcan way of life".
The following year, Spock returned to Romulus and the group decided to continue their peaceful ways of achieving their goals. Sela blamed the movement for the death of Tal'aura in 2384. It was thought that this may eventually lead to war. (ST website: The Path to 2409)
Following the Hobus supernova, Kererek, a Romulan naval officer, reached out to D'Tan, a prominent Unificationist leader, in hopes of finding a way to counter the growing influence of the Tal Shiar in the remnants of the Empire. The two forged an alliance that grew into the Romulan Republic, which in 2409 was recognized by the Federation and the Klingon Empire as an independent state. (STO missions: "Voice of the Voiceless", "Turning Point")