If nothing else, Rogue One: A Star Wars Story seems to be tying the Star Wars franchise together in some interesting ways. It bridges Revenge of the Sith and A New Hope, it connects The Clone Wars and Star Wars: Rebels to the live-action films and it gives us a glimpse of what's going on in the Star Wars universe beyond the scope of Luke Skywalker and his adventures. It makes these connections by including characters from the various properties, including an aged live-action version of Clone Wars character Saw Gerrera (played by Forest Whitaker). This is pretty significant, since it establishes in a very real way the canonical nature of the animated Star Wars properties.
If you didn't follow The Clone Wars, however, you might not know exactly who Saw Gerrera is. Here's everything you need to know about his backstory and his connection to the Rebel Alliance heading into Rogue One.
While Saw Gerrera first appeared in season 5 of The Clone Wars, he was originally planned as a character from George Lucas' live-action Star Wars TV show that never got off the ground. When that show failed to launch, the character was worked in to the animated series as an Onderonian rebel who fought against the King of Onderon's pledge of loyalty to the Separatists. Alongside his sister Steela, and with the support of Obi-Wan Kenobi, Anakin Skywalker and his Padawan Ahsoka Tano, Saw and the Onderon rebels waged a guerrilla war against King Sanjay Rash and his Separatist allies.
Saw featured prominently in a four-episode story arc that detailed his rebel group's successful overthrow of King Rash and the restoration of the previous king to the throne. This was more than just a victory for the Republic against the Separatists, however; the defeat of Rash established that Saw and his rebels would not stand for an oppressive ruler and would actively fight to depose one. It also establishes Onderon as one of the first locations in the Star Wars universe to experience open rebellion within the timespan of the films themselves.
Saw was not a natural leader; while he and his rebels were being trained by the Jedi, his rivalry with one of the other rebels disrupted the training and almost brought the two men to blows. While Saw was a brave man and a formidable soldier, he was angry and intent on taking the battle to the enemy instead of taking the time to formulate a plan of attack. This was one of his biggest challenges, since his attitude was a major hindrance to the group being able to work together. It wasn't until Separatist droids located the rebel base and Saw's group had its first real taste of battle that he started developing trust in his fellow soldiers. They began to work as a team, under the leadership of Saw's sister.
Once the Jedi left Onderon, Saw and his rebels continued the fight against the Separatist armies in hopes of restoring the rightful ruler of Onderon. The rebels succeeded, but at a heavy price; Steela was killed in the final confrontation, leaving Saw to take command of the rebels and lead them to victory. He was a much stronger leader now, but the loss of his sister weighed heavily on him as it was his own recklessness that got her killed.
The rebels continued fighting for the freedom of Onderon while the Clone Wars continued. Continuing to use guerilla tactics, they helped ensure that Separatist forces couldn't secure another foothold on the planet. Saw's reputation as a leader grew, as did the reputation of his rebel group. The group was held in enough esteem on Onderon that before the rise of the Empire, one of Saw's rebels (Lux Bonteri, the man that Saw almost came to blows with early on) represented the planet as a member of the Galactic Senate.
The Rebellion Continues
When the Republic fell and was replaced by the Galactic Empire, Saw and his rebels viewed this as a major threat to their way of life. His rebels grew into a larger militia group known as the Partisans, and became a major thorn in the Empire's side. The Partisans were fierce, often leaving casualties in their wake; at one point, the Empire even used them as part of a propaganda campaign to try and paint the entire Rebel Alliance as a terrorist group.
In addition to using guerrilla tactics and its signature brutality, the Partisans hired mercenaries as well to bring the fight to the Empire. An episode of Star Wars: Rebels featured Agent Kallus recounting an encounter with a Lasat mercenary that worked for Saw who killed most of Kallus' unit (though he himself had obviously survived).
The Partisans' reputation for brutality actually lasted longer than the Empire itself. Saw and the Partisans were mentioned by New Republic senator Ransolm Casterfo in Star Wars: Bloodline as an example of the so-called "terrorist tactics" of the rebellion during the war against the Empire.
The First Rebels
When Saw Gerrera was first introduced, the eventual intention was for his group of rebels to be one of the initial cells of what would eventually become the Rebel Alliance. The original plan was for the Partisans and other militias to come together, joining forces against the Empire and over time growing into the alliance that fans knew from the Star Wars movies. The Partisans may still play some part in the formation of the Rebel Alliance, though it looks like they may be a bit more separate than some of the other groups. There certainly haven't been any rebel groups seen in the movies that actively participate in attacks as brutal as those of the Partisans, and Saw's audio from the Rogue One trailers suggests that he may be more paranoid than many of the rebel leaders we've seen thus far.
Still, Saw Gerrera and his rebels hold an important position in the battle against the Empire. Despite Bail Organa and other rebel leaders being more prominent in organizing the Rebel Alliance into the "good guys" we know from the movies, Saw and his troops were some of the first to openly rebel against the Empire and hurt the Empire every chance they had. If it wasn't for Saw Gerrera and his rebels from Onderon, the larger rebellion that eventually defeated the Galactic Empire may not have ever come together.