When we again see Jean-Luc Picard for the first time since 2002's Star trek: Nemesis in the premiere episode of Star Trek: Picard, we learn a key piece of information about his character (other than the fact he's still hung up on Data): He has abandoned his service in Starfleet. That's a pretty big deal: Picard is a guy who spent his entire life forwarding the ideals of humanity's exploration of space as captain of the Enterprise, who wanted to be a Starfleet captain from childhood, and who sacrificed the prospect of having a family for his career. For Picard to leave behind the life of an explorer behind suggests something major must have happened.
In the first episode of Star Trek: Picard, "Remembrance," we get some sense of what happened to drive the legendary captain, and later admiral, from Starfleet service. We don't have all the details yet, but some exposition in "Remembrance" explains the broad strokes of what finally caused Picard to leave space and head back to his family's vineyard in France. Put simply: Starfleet let Picard down.
Star Trek: Picard News and Features
The situation started when the Romulan Star Empire learned a star in its territory was going to explode in a supernova, destroying Romulus and killing a whole bunch of Romulans. That's actually the setup for J.J. Abrams 2009 Star Trek movie, and while most of that movie takes place in an alternate timeline, the events that destroy Romulus are part of the prime Star Trek timeline where Star Trek: The Next Generation, its four sequel movies, and Star Trek: Picard take place.
With impending doom bearing down on them, the Romulans reached out to the Federation for help. The thing is, the Romulans and the Federation have been enemies for a long, long time. Throughout The Next Generation, there's an uneasy cease-fire between the two factions, but the Romulans always seem to be on the verge of war and they're constantly spying on everyone around them. In fact, Star Trek: Nemesis is all about how the Enterprise crew narrowly stops a Romulan leader from destroying Earth and starting a war with the Federation. So basically up to learning about the supernova, tensions between the Romulans and the Federation are relatively high.
So when the Romulans asked for help, there were a lot of people in the Federation who were reluctant to expend a bunch of resources and maybe risk a lot of lives in order to help their enemies. But the tenets of the Federation and Starfleet are all about respect for life and the duty to help others--so Picard manages to convince Starfleet to mount a rescue to save the Romulans, despite their history, because it's the right thing to do.
Starfleet builds a giant rescue armada of ships at the Utopia Planitia shipyard orbiting Mars colony, and everything's going pretty well. Then tragedy strikes--a group of rogue "synths," or robotic workers, attacks Mars. The colony is utterly destroyed, as is the rescue armada.
As a result, the Federation bans all synths, and Starfleet decides not to mount the rescue of Romulus. As Picard says, he believes that not helping the Romulans demonstrates the Federation and Starfleet turning its back on its duties and principles, and that caused him to resign.
We don't know much else about the details of Picard's departure, although the Romulans who work in the Picard Chateau, Laris and Zhaban, make it clear that a lot of Romulans respect Picard for what he did. Unfortunately, Picard's decision took him out of Starfleet, and it's clear in the first episode of the show that he regrets the decision. We'll have to wait for future episodes to expand on Picard's backstory even more.
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