The Return
During the final season of The Clone Wars, we saw that after leaving the Jedi Order, Ahsoka had been living a lie. Attempting to exist, while also hiding her powers. In this, she was moderately successful because she was still too heroic to do nothing in dire circumstances. So when she ran into Bo-Katan again, Ahsoka once again found herself allying with the Jedi and the Republic (albeit as a citizen) to help them defend Mandalore.
As Ahsoka, Bo-Katan and Rex attempted to defend Mandalore from Maul’s forces (happening at the same time as events in Revenge of the Sith), a few key things happen. One, Maul reveals to her that Anakin has been groomed by Palpatine to become his apprentice, Darth Vader. Two, Palpatine puts Order 66 into effect, killing almost all the Jedi in the process. Ahsoka does make it out safely though, but just barely.
