The re-unification of the Vulcan and Romulan peoples has been a subject of interest on both Vulcan and Romulus. While the Vulcans were open to peaceful re-unification, the Romulans' interest in the matter has proven to be one of conquest. (ENT: "Kir'Shara"; TNG: "Unification II")
Divergence with Vulcan
The Vulcans were once an extremely violent and emotional people, waging almost constant warfare on each other. As their level of technology improved, the Vulcans' violent nature threatened the species with extinction.
A group of dissenters, known as "those who march beneath the raptor's wings", did not accept Surak's teachings, and left the planet Vulcan at this time. This group later settled on the planets Romulus and Remus and become the Romulan species, eventually forming a vast and powerful interstellar empire. (TOS: "Balance of Terror", "The Savage Curtain"; ENT: "Awakening", "Kir'Shara")
22nd century origins
Sometime in the mid-22nd century, Administrator V'Las became the head of the Vulcan High Command. Unknown to most at the time, V'Las had been working with a Romulan operative, Major Talok, to pave the way for the conquest of Vulcan by the Romulan Star Empire.
In 2154, the Earth embassy on Vulcan was bombed. V'Las placed the blame on the Syrrannites, a group of Vulcans dedicated to returning Vulcan to the true teachings of Surak. An investigation by Starfleet officers Captain Jonathan Archer and Commander T'Pol of the starship Enterprise NX-01 revealed the Syrrannites' innocence in the bombing. It was determined that V'Las, in an attempt to eliminate the Syrrannites as a political obstacle in his plans to attack Andoria, had one of his aides bomb the embassy and plant evidence implicating them, allowing V'Las to eliminate the Syrrannites without incident or opposition.
Unfortunately for V'Las, these plans fell through when Archer and T'Pau, the leader of the Syrrannite movement, brought the Kir'Shara before the Vulcan High Command, bringing forth the Vulcan Reformation. V'Las was expelled from his position, although the involvement of Major Talok remained secret. Talok was forced to return to Romulus following these events, telling V'Las that the re-unification of the Vulcan and Romulan peoples would have to wait. (ENT: "The Forge", "Awakening", "Kir'Shara")
Following these events, the Romulans focused their concentration on their campaign to destabilize the Alpha Quadrant's major powers, and later their war with Earth. The Romulans' plans for reunification were further delayed by the Empire's isolationist policies following the end of the war in 2160 through the mid-23rd century. (ENT: "Babel One", "United", "The Aenar"; TOS: "Balance of Terror")
In 2368, the celebrated Federation ambassador and former Starfleet officer Spock traveled to Romulus at the invitation of Romulan senator Pardek. The new proconsul of the Romulan Senate, Neral, had been promising reform and Pardek believed he could be convinced to join the re-unification movement.
Spock left without notifying the Federation, and in response Captain Jean-Luc Picard and Lieutenant Commander Data of the USS Enterprise were sent to determine what he was doing there and ensure that he had not defected to the Romulan Empire.
Despite a promising meeting with Neral, in which he promised public endorsement of re-unification with Vulcan, Spock and Picard suspected foul play. Their suspicions proved correct when they were taken into custody by Commander Sela. The Romulans' plan was to have Spock deliver a speech appealing to Vulcan to welcome a "peace envoy" from Romulus, only to have the incoming convoy filled with Romulan troops ready to conquer Vulcan. Spock refused, and Sela decided to use a holographic representation of him to deliver the speech. Data and Spock were able to access the computer system and reprogram the hologram to warn the Federation of the invasion. When the Enterprise moved to intercept the invasion force, a Romulan Warbird decloaked and destroyed it, preventing their capture. Picard and Data returned to Federation space shortly after, although Spock chose to remain behind and continue his efforts of promoting peaceful re-unification of Vulcan and Romulus. (TNG: "Unification I", "Unification II")