Rebellion Era Campaign
Alliance to Restore the Republic
The very core origins of the Alliance to Restore the Republic (also known as Rebel Alliance) dates back to 19 BBY when a few Senators, including Bail Organa and Mon Mothma, joined the Delegation of the 2000 in their opposition against the ever-growing authority and power handed over to Chancellor Palpatine.
Ever since the formation of the Galatic Empire, many anti-Imperial groups of different sizes (factions, planets, systems) have appeared throughout the galaxy. In a plot to force the rebels out of hiding, Darth Vader sent his secret apprentice, Starkiller, to earn the trust of these rebels and encourage them to form a combined and organized rebel force.
Betrayed by his master, Starkiller ended up emphatising with the rebels, and even sacrificed his own life in order to save theirs. In 2 BBY, the Treaty of Corellia was signed, which marked the official formation of the Alliance. This sparked an open civil war against the Empire all throughout the galaxy.