Vulcan reunification

From Memory Alpha, the free Star Trek reference.

The Vulcan reunification was the planned reunification of the Vulcan and Romulan civilizations.

Vulcan Administrator V'Las and a Romulan who posed as the Vulcan Major Talok secretly worked towards a reunification of their two cultures, before being thwarted by the Vulcan Reformation. (ENT: "Kir'Shara")

However, with the teachings of Surak rediscovered and formation of the Federation, the reunification of Vulcan and Romulus would never be realized. (ENT: "These Are the Voyages...")

The Romulan underground movement supported the idea of reunification. Senator Pardek infiltrated this movement in an elaborate plot to invade Vulcan. (TNG: "Unification II") Ambassador Spock joined this movement and it would continue to support the idea of reunification for many years to come. (TNG: "Unification I", "Face of the Enemy"; DS9: "Image in the Sand")