For all you "Star Wars" fans, we've got something for you before the release of the upcoming "Rogue One - A Star Wars Story." What started the rebellion? What events led from the end of "Episode III: Revenge of the Sith" to the beginning of "Episode IV: A New Hope?"
There were numerous "Star Wars" materials that tackled on the issue of the events that led up to "A New Hope." One of which is the recent "Star Wars: Ahsoka" novel by E.K. Johnston features Ahsoka Tano (the Jedi apprentice of Anakin Skywalker from the "Clone Wars" animated series) and Bail Organa meeting up together to secretely form the Rebellion movement, according to The Hollywood Reporter. Another novel called "Star Wars: Catalyst - A Rogue One Story" by James Luceno, even goes further back on the origins of the Rebels in the entire "Star Wars" timeline.
"'Catalyst' begins a few months after the start of the Clone Wars, and the Separatists referred to in the [publisher's official] description are in fact aligned with the Confederacy of Independent Systems," Luceno told The Verge, as acknowledged the conflict and the antagonistic factions seen in "Episode II: Attack of the Clones" and "Episode III: Revenge of the Sith."
The author continued on: "A direct link can be made between the Rebellion and the former Separatists, who, in the wake of the Republic victory, never received reparations promised by the Empire, and remained ostracized - and in many cases occupied - while the Empire spread its tentacles far and wide. Instead of fairness, the galaxy fell victim to fear of the Empire's ever-expanding military. The absence of the Jedi Order - their fire gone out of the universe - left many worlds without any recourses to counter injustices."
What the author meant to say was that the Separatists in his novel "Catalyst" had a direct and essential connection to the events of "Rogue One," especially with Galen Erso (Mads Mikkelsen) and his daughter Jyn Erso (Felicity Jones), who's also the protagonist of the upcoming movie.
Perhaps the "Rogue One - A Star Wars Story" would explain the backstory a little more once it premieres, but it would be worth having a look at those novels for immersion.