after the clone wars ended, The Republic wasn't turned into the empire, Palpatine was executed, Anakin didn't turn to the dark side, padme was alive and had the kids Luke and Leia, it was 20 years later they detected the SOS signal from a unknown planet called Earth, the broadcast was named UA Class 1A, the jedi council ordered the 501st Legion and the 212th Attack Battalion to head to Earth alongside their fleet, Luke joined his Father after being knighted to be in the 501st