Spock (Leonard Nimoy) spent the latter years of his life trying to unify the Vulcans and Romulans, and the reasons why were set up by Romulan Star Empire in the Beta Quadrant. The Romulans have been enemies of the United Federation of Planets for centuries, but Spock's work eventually did change Vulcan and Romulan relations, although it took hundreds of years after the Vulcan hero's death.
By tracking Spock's interactions with Romulans in Star Trek: The Original Series, the root of his desire to unify the Romulans and Vulcans in the 24th century becomes clearer. Spock had key incidents with Romulans that sparked his interest in unification, and this includes the Klingon peace talks in Star Trek VI: The Undiscovered Country. By the 24th century, Ambassador Spock lived on Romulus and labored underground toward his goal of bringing the Vulcans and Romulans together. In the classic J.J. Abrams' Star Trek 2009, Spock attempted to save the Romulans from their sun going supernova, but he failed and was permanently stuck in the 23rd-century Kelvin Timeline. Tragically, Spock didn't live to see his dream become reality.
How Spock's Romulan Missions In TOS Set Up His Unification Dream
In the Star Trek: TOS season 1 episode, "Balance of Terror," Spock was as shocked as anyone on the Starship Enterprise to see that Romulans were practically identical to Vulcans. Spock even experienced a racist attitude from one of his crew mates whose family died fighting in the Romulan War. Although he didn't vocalize it at the time, Spock must have been curious to know more about the Romulans. The Vulcan Science Officer's chance to meet Romulans firsthand came in the TOS season 3 episode, "The Enterprise Incident," when Captain Kirk (William Shatner) enacted a complex espionage mission to steal a Romulan cloaking device while Spock distracted and seduced the Romulan Commander (Joanne Linville).
Spock didn't have significant interactions with Romulans otherwise, but it's safe to conjecture his brief time romancing the Romulan Commander fed his desire to bring the Vulcans and Romulans together. The next important step in Spock's mission was in Star Trek VI: The Undiscovered Country. Not only did Spock become a diplomat who brokered the Federation's peace negotiations with the Klingons' Chancellor Gorkon (David Warner), but his own Vulcan protege, Valeris (Kim Cattrall) sided with conspirators, including Romulan Ambassador Nanclus (Darryl Henriques), to stop the peace talks via murder. In Star Trek VI, Spock came to understand that diplomacy was the key to Vulcan and Romulan unification, and this became the focus of his post-Starfleet career after Kirk was believed dead in 2293.
Spock's Unification Dream Happened In Star Trek: Discovery Season 3
Tragically, Spock remained in the Kelvin Timeline for the remainder of his life, and the Vulcan died (for the second and last time) on January 2, 2263 - 124 years in the past from his 24th-century starting point. But Spock's dream eventually happened as his adopted sister, Michael Burnham (Sonequa Martin-Green), learned in Vulcan and Romulan unification come true over 900 years after Spock vanished from Star Trek's Prime Timeline.