Worf (Michael Dorn) joining Starfleet was previously set up in the Star Trek: The Original Series episode "Errand of Mercy". In that episode, tensions between the Klingon Empire and the Federation have once again become frayed, and it appears to be only a matter of time until the two galactic powers are at war once again. However, due to the intervention of a neutral and benevolent species called the Organians, war is averted. Before Kirk, Spock, and the Klingons leave the Organian's world, they're told that one day the Federation and the Empire will be allies, which sets up Worf's entry into Starfleet.
Michael Dorn's Worf is a much-loved character who first debuted in Star Trek: The Next Generation and later joined Star Trek: Deep Space Nine as a series regular. Orphaned and raised by a human family, Worf became the first Klingon to join Starfleet, something made possible by the peace that existed between the Federation and the Klingon Empire. Worf is soon to return in Star Trek: Picard season 3, as a man of peace who joins Jean-Luc's mission to save the galaxy one last time. This proves that, as predicted by the Organians, the Klingons and the Federation are still firm allies.
Why Worf Was The First Klingon In Starfleet
Star Trek creator Gene Roddenberry intended Worf to represent the peace that existed between the Federation and the Klingon Empire. In TNG season 6, episodes 16 and 17, it's revealed that he joined Starfleet because of a vision that told the young Worf he would do something no Klingon had ever done before. Originally, Worf wasn't meant to be a series regular, but Michael Dorn impressed the team so much that his role was expanded. So impressive was Michael Dorn that Nicholas Meyer cast him as Worf's grandfather in Star Trek VI: The Undiscovered Country, who defended Kirk and McCoy from allegations of assassinating the Klingon Chancellor.
By the time Star Trek 6 was released, TNG was in its fifth season, and Worf was one of the show's most beloved characters. Giving Worf's grandfather a vitally important role in the Klingon peace process was a nod to the character's importance to Star Trek canon. Interestingly, one of the most important Worf arcs, seasons 4 and 5 episodes 26 and 1, "Redemption" was almost abandoned by Roddenberry because he didn't feel that a Klingon should be a central character. Despite his initial intentions, it's clear that Roddenberry wasn't as progressive when it came to the Klingons as Starfleet was.
Why The Klingons And The Federation Became Allies
After over four years of TNG, the circumstances of the peace process between the Klingon Empire and the Federation were finally explored in Star Trek 6. Co-written by Leonard Nimoy, Undiscovered Country was an allegory for the changing relationship between the Soviet Union and the West in the wake of the Chernobyl disaster. Despite his disagreement with the more militaristic and racist elements of the movie, this plotline tied in with Roddenberry's vision for a peaceful future between the Federation and the Klingon Empire.
Following the destruction of one of their moons, Qo'nos, the Klingon homeworld faced a doomed future. Spock opened a diplomatic dialog to offer support to the Klingon Empire, finally bringing an end to the hostility between the two powers. Not everyone in Starfleet and the Klingon Empire was behind this course of action, and actively work against the peace process and attempted to sabotage it. In their final big-screen outing, the Star Trek: The Original Series crew averted disaster, ensuring the peace treaty was signed and safeguarding the future of both Starfleet and the Klingon Empire, as predicted by the Organians.