"You fought in the rebellion against the Empire?"
-Luke Skywalker, to C-3PO
The Alliance to Restore the Republic was an faction formed out of The rebellion at some point before 1 BBY and the Battle of Yavin. It was also formed out of several Separatist Holdouts, members of the Delegation of 2,000, and a few surving Jedi Knights like Kanan Jarrus and Ben Kenobi. The Alliance was made up of mostly volenteers who wanted to put an end to the Empire's regin of fear across the galaxy, but also consisted of some old Clone Wars technology like the AT-RT Walkers and some reprogramed B1 Battle Droid and Hailfire droid units. The Alliance would gain support from several Imperial-hating factions for years until it reformed itself into the New Republic in 4 ABY shortly after the Battle of Endor. A few members of the Rebel Alliance would join Princess Leia Organa in fighting the First Order in the Resistance.