Some Starfleet decisions resonate throughout the franchise more than others, leaving a legacy felt across multiple series and films for years, even decades. These decisions, with their profound impact, sometimes rewrite the “Star Trek” universe in ways not immediately apparent. Whether they emerge as retcons or are crafted as narrative catalysts to redirect the series’ trajectory, their effects are enduring, fundamentally transforming the franchise.

From seemingly simple choices to explore uncharted realms to the monumental decisions of war or peace, here are ten pivotal Starfleet decisions that have left an indelible mark on the “Star Trek” franchise.

The introduction of the Cardassians in “Star Trek: The Next Generation” episode “The Wounded” was somewhat of a retcon. Portrayed as cunning antagonists, they were revealed to have been embroiled in a prolonged, brutal conflict with the Federation prior to the episode’s timeline. Although a peace treaty concluded the war, tensions simmered beneath the surface, leading to a cold war scenario. This treaty’s concessions resulted in the emergence of the Maquis, a rogue group of former Federation colonists who, unwilling to leave their homes, found themselves residing in Cardassian territory post-treaty.

The Cardassians were a bit of a retcon when they appeared for the first time in the “Star Trek: The Next Generation” episode “The Wounded.” They are introduced as a race of insidious villains who’d been engaged in a long and bloody war with the Federation years before the events of the episode. A peace treaty ended the war, but the conflict never really died. The episode reveals that it simply led to a cold war of sorts, with the Cardassians seemingly preparing for renewed hostilities. But later, it’s revealed that the concession of key territories in the treaty led to a rogue group called the Maquis, made up of former Federation colonists who refused to abandon their homes and found themselves living in Cardassian space after the treaty.

This storyline officially begins in the “TNG” episode “Journey’s End” and continues in “Star Trek: Deep Space Nine,” with civilians becoming terrorists to fight their Cardassian rivals in the demilitarized zone between the Cardassian Empire and the Federation. Not only do the Maquis play a major role in “DS9” for years, even helping to push the Cardassians into an alliance with the Dominion, but it’s also on a mission to stop the Maquis that the USS Voyager is sent halfway across the galaxy in that show’s premiere episode. And the exploration of the Delta Quadrant by Captain Janeway and Voyager — not to mention the decimation of the Borg they find there, with the help of new recruit Seven of Nine — leads to the events of “Star Trek: Picard” and even the CGI-animated series, “Star Trek: Prodigy.”

There might be no single Starfleet decision that had a more profound impact than the one made at the outset of “Star Trek VI: The Undiscovered Country.” It’s there that we learn that the vaunted Klingon Empire, which had battled the Federation for decades, was on the precipice of self-destruction after the decimation of their homeworld’s moon, Praxis. While some, including Captain Kirk, would prefer to let the Klingons implode, Starfleet and the Federation instead choose to make an overture of peace, extending an olive branch by offering to end their war and help save their dying empire.

In choosing to show mercy to their struggling enemy, the Federation redraws the face of the Alpha Quadrant forever, erasing the Klingon/Federation Neutral Zone, and turning the two biggest powers in the “Star Trek” universe from bitter enemies to friendly allies. Eventually, the Klingon/Federation alliance is strengthened thanks to the efforts of the USS Enterprise-C at Nerendra III, and before long, there’s even a Klingon serving in Starfleet with Worf on the bridge of the Enterprise-D. 

The Federation’s new, friendly relationship with the Klingons also prompts their other major enemy, the Romulans, to essentially go into recluse mode for years. And when they return, they are an even greater threat whose sole mission is to sabotage the Klingon/Federation alliance, which they see as the only roadblock to interstellar supremacy. 

Perhaps surprisingly, there are a handful of impactful Starfleet decisions that originate in parallel realities. One such decision occurs in the “Next Generation” installment “Yesterday’s Enterprise,” which begins with the emergence of the Enterprise-C from a spatial anomaly, a ship supposedly destroyed two decades earlier in a historic battle that helped secure the aforementioned Klingon/Federation alliance. And its appearance creates a new, alternate timeline in which the Enterprise-D’s Lt. Yar didn’t die too soon in the Season 1 episode “Skin of Evil.”

In “Yesterday’s Enterprise,” this alternate version of Yar learns from the enigmatic Guinan that she died in the Prime Timeline. Looking for a more honorable death, Yar gets Picard’s permission to join the Enterprise-C when it returns to the past to restore history. There, she hopes to sacrifice her life to help the ship fend off an important (but ultimately fatal) attack by Romulans in an effort to restore the timeline. A year later, in the two-part cliffhanger “Redemption,” we learn that the alternate Yar survived when she went back in time and had a baby with a Romulan officer while held as a prisoner of war.

Yar’s baby, of course, grew up to become Commander Sela, a devious Romulan military leader who leads multiple attacks on the Federation. She spearheads a secret alliance between the Romulan Empire and the House of Duras, who seek to wrest control of the Klingon Empire. Later, she nearly succeeds in conquering Vulcan — and all because Picard thought Yar should have a more noble death.

One of the best episodes of “Star Trek: The Next Generation” is the Season 2 entry “Measure of a Man,” a powerful story that sees Captain Picard defending the android Commander Data in a fight for equal rights. But the entire episode hinges on Starfleet’s decision to try to create more beings like Data, with scientist Bruce Maddox hoping to replicate the android’s positronic brain. Little did anyone realize at the time, though, that this decision would alter “Star Trek” forever.

For starters, Picard’s rigorous defense of Data in “Measure of a Man” led to the issue of rights for sentient computers becoming a hot-button issue within the franchise: it’s further explored in other “TNG” episodes like Season 3’s “The Offspring” and Season 6’s “The Quality of Life.” Later, the issue receives even more attention on “Star Trek: Voyager” where the holographic Doctor argues for similar rights — and for other holograms of his ilk — which even leads to the formation of an entire holographic race of people in the episode “Flesh and Blood.”

But the biggest consequence of Maddox’s research is seen decades later in “Star Trek: Picard,” where we learn that Maddox eventually succeeded in duplicating Data. And he helped create a race of synthetic people who became Starfleet laborers, ignoring the rights that Picard and Data fought so hard for. Eventually, those laborers revolted, leading to a ban on android technology while quashing Starfleet’s developing plan to rescue the Romulans when their planet’s destruction was imminent. Which leads us to…

In 2009, JJ Abrams rebooted the “Star Trek” franchise, and that one decision by Paramount reshaped the franchise overnight. But wisely, Abrams didn’t just erase all existing continuity: Instead, he created an all-new timeline as a platform to tell stories unencumbered by decades of complex continuity. Years later, it’s revealed in “Star Trek: Picard” that this alternate timeline, known as the Kelvin Timeline, only exists because of Starfleet’s disastrous decision not to help save the people of Romulus before its destruction at the hands of a supernova.

While it’s true that the synth uprising played a critical role in Starfleet’s decision to abandon plans to save the Romulan people, they were never overly fond of the idea to begin with. It took intense lobbying from Admiral Picard to convince them, and they only begrudgingly agreed. Even in the wake of the synth uprising that ultimately destroyed the rescue armada, there was still time to mount some kind of evacuation of Romulus, but Starfleet and the Federation declined. In the end, this lack of action led one Romulan, Nero, to embark on a mission of revenge against Ambassador Spock, who’d promised that Starfleet would save his people.

This vengeful mission is the one that sent him on a trip back in time, where he killed the father of James T. Kirk, creating the Kelvin Timeline. Looking back, if Starfleet had attempted to rescue the Romulans, the entire JJ Abrams trilogy might never have occurred.

Thanks to the cosmic trickster known as Q, Starfleet had its first confrontation with the Borg in the “Next Generation” episode “Q Who.” But it was in one of the greatest season-ending cliffhangers in TV history, “The Best of Both Worlds,” that the Borg became the Federation’s greatest foe. Later, in the “TNG” episode “I, Borg,” the Enterprise rescues a damaged Borg drone, hoping to learn more about their technology, and winds up discovering a way to cripple the entire Borg collective with a devastating computer virus of sorts. But at the climax of the episode, Captain Picard makes the bold decision to spare the Borg and returns the drone back to its people.

At the time, the likes of Guinan and Dr. Crusher successfully argued that the Borg aren’t monsters; they are innocent people who’ve been perverted by the Borg’s technology. They argue that even in their Borg state, they are simply acting out their programming, and infecting them with a virus would be crossing the line. This convinces Picard that, while the Borg are a ruthless enemy, they deserve mercy. 

In the end, however, not going through with the plan — which likely would have decimated the Borg — leads to their continued quest to conquer Earth. This quest inflicts untold deaths, more conquered worlds — and a final confrontation with the Borg Queen in “Star Trek: Picard.” It’s an important moment that demonstrates how showing mercy to those who want nothing less than your extermination is not always the wisest course.

“Star Trek: Deep Space Nine” began its journey in 1993, envisioned as a different kind of series that’s set on a space station instead of a starship. To make it even more unique, the crew of the station discover a wormhole in the show’s opening episode that leads to the distant and unexplored Gamma Quadrant. At the time, Starfleet saw the wormhole as a key to exploration, but it wound up being the catalyst for a protracted interstellar war with a vast alien empire that took countless lives and influenced “Star Trek” for three decades.

As Starfleet begins exploring the Gamma Quadrant with the aid of the wormhole, they find it ripe for trade and scientific discovery, but they (and other Alpha Quadrant powers like the Ferengi) seem to pay no heed to the people of this distant region of space, and all but ignore warnings of a cosmic power known as the Dominion. Rather than seeking out contact with this empire and learning more about them before proceeding, Starfleet makes the reckless decision to explore the Gamma Quadrant while fully aware that they are violating the Dominion’s territory.

As we all know now, the Dominion were not welcoming, and in light of Starfleet’s operations in their space, they opted to go to war, intent on conquering the Alpha Quadrant. And even after a five-year battle that cost hundreds of millions of lives, members of the Dominion survived, with the Changeling Vadic returning to help the Borg Queen in her plot to assimilate Earth in Season 3 of “Star Trek: Picard.”

We’ve already covered the decision that led to the existence of Commander Sela, the Romulan daughter of Lt. Yar from an alternate timeline. But in the two-part episode “Redemption” that sees Sela allying with the Klingon House of Duras, another decision is made that had ramifications for far longer than anyone imagined. When Captain Picard gets wind of the Romulan plot to aid a Klingon civil war, he decides to carry out a plan to expose the Romulans.

The plan, developed thanks to previous conflicts with the House of Duras, doesn’t involve any direct combat, but instead hopes to reveal cloaked Romulan vessels as they traverse an energy field created by a fleet of Starfleet ships stationed between Klingon and Romulan territories. Those Romulan vessels are commanded by Sela, and if it weren’t for Picard’s decision to help the newly-installed Klingon Chancellor Gowron, the House of Duras may very well have usurped the Klingon Empire and led a new Romulan/Klingon alliance.

Thanks to Picard’s intervention, however, not only does Gowron come to power, he becomes fond of the Enterprise captain — and the ship’s stalwart Klingon security chief Mr. Worf. This leads to a closer relationship between the Federation and the Klingons, the opposite of what the Romulans and Commander Sela had intended. Getting involved in the Klingon civil war is ultimately one of Picard’s best decisions, with an impact still felt in today’s newest “Star Trek” stories.

In the 2001 prequel series “Star Trek: Enterprise,” we learn that it took time, and presumably some precarious missions, to teach Starfleet brass to institute the Prime Directive, which prohibits Starfleet ships from making contact with primitive civilizations. In the episode “Dear Doctor,” Captain Archer is faced with a terrifying ethical dilemma when the NX-01 comes across a people begging them for medical help, as a pandemic ravages their world. Though Captain Archer feels compelled to help, the Enterprise’s Dr. Phlox discovers that there’s another race of even more primitive humanoids on the planet who stand to evolve into the dominant species if the pandemic is left to do its thing.

Unfortunately for Archer, there is no Prime Directive yet, a directive that would have demanded the Enterprise not get involved, to allow nature to take its course in a pre-warp civilization. We’re left to believe Starfleet’s later decision to create the Prime Directive was the result of missions like this, and enacting that directive alters the fundamental nature of Starfleet itself forever. 

Since Starfleet established the Prime Directive, it’s been the basis for numerous stories, many of which have become all-time classics. It was at the center of the eighth feature film, “Star Trek: Insurrection,” while some of those stories altered the future of “Star Trek” themselves. But more than just how the Prime Directive shaped episodes and movies, it altered the “Star Trek” universe, as it dictated the fate of numerous worlds, from Mintaka III to Kiley 279.

When the Borg attacked Earth under the command of a corrupted Jean-Luc Picard in “The Best of Both Worlds,” Starfleet wasn’t at all prepared. They did, however, order a task force of 40 Starfleet ships to cut off the Borg attack at Wolf 359, a star system not far from Earth. Tragically, nearly every Starfleet ship was destroyed in the battle, leading to more than 10,000 casualties. Though Starfleet had little choice, the decision to confront the Borg at Wolf 359 left lasting effects on the franchise that were felt across multiple sequels and spin-offs.

The biggest impact, of course, was on Captain Picard himself, who had to live with the guilt of having led the Borg in the attack, even if he wasn’t in control of his own mind. A number of later “TNG” stories were told under the shadow of the Battle of Wolf 359, including the film “Star Trek: First Contact.” The battle also prompts Starfleet to draft plans for an entirely new fleet of warships, which leads to the creation of the USS Defiant in “Deep Space Nine.” And it was that battle that forever altered the fate of Commander Benjamin Sisko, who survived but lost his wife in the assault before he was assigned to station Deep Space Nine. 

Sisko wasn’t the only Starfleet officer scarred by the Battle of Wolf 359, of course. In addition, a young engineer named Liam Shaw managed to live through the attack as well. But his survivor’s guilt weighed on him for years, leading to a bitter confrontation with Jean-Luc Picard in Season 3 of “Star Trek: Picard,” fueled by his hatred of the Borg.