A rebel cell was established by Senator Mon Mothma of Chandrila in 6 BBY.
By 5 BBY, Mon Mothma's rebel cell had joined with Senator Bail Organa's resistance movement to form a core element of the nascent Alliance to Restore the Republic, which at that time was referred to as simply the "Alliance" or the "Rebellion" by Mothma.
In 2 BBY, Mothma defected from the Imperial Senate and publicly declared her allegiance to the rebellion against the Galactic Empire. This declaration sparked the official formation of the Alliance to Restore the Republic, of which Mothma became the leader.