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In the last years of the Galactic Republic, Mon Mothma represented Chandrila in the Senate. Though a Loyalist Senator, Mothma worried about Supreme Chancellor Palpatine’s accumulation of power and the militarization of the Republic.
In the final months of the Clone Wars, Mothma and Organa laid the groundwork for resistance to Palpatine’s rule, fearing that he would refuse to give up the emergency powers granted during the fight against the Separatists. After Palpatine declared himself Emperor, Mothma remained in the Senate, but worked in secret with Organa and other operatives including Luthen Rael and her cousin, Vel Sartha, to unite rebel factions into an Alliance to restore the Republic.
Mon Mothma became an enemy of the Empire following her public condemnation of Palpatine. It was her final act as senator, but also her first act in truly organizing a rebellion. Shortly after, the Ghost crew escorted Mothma to a secret location; there, she put out a call for other rebel cells to join her. A flurry of ships came to her signal, and the Rebel Alliance was born.
In the years ahead, Mothma would shepherd the Rebel Alliance from a secret base on Yavin 4. But the need to strike strongly against the Empire became urgent when the Alliance learned of an Imperial superweapon. Mothma tasked Jyn Erso, daughter of Death Star weapons architect Galen Erso, to contact extremist rebel Saw Gerrera, as well as her father, in an effort to discover a weakness in the battle station. Ultimately, the leading council of the Rebellion would vote not to attempt a covert operation to secure the plans; Erso, with a small band of rebels, took matters into their own hands and went to Scarif, home of the Imperial data center that housed the Death Star schematics. When Mothma discovered this deception, she did not block it. Instead, she readied the rebel fleet to support Erso and receive the data transmission, resulting in the plans coming into the care of Princess Leia Organa.
In rapid succession, the Emperor disbanded the Senate and Mothma's ally, Bail Organa died when the Death Star destroyed Alderaan. Mothma went into hiding, serving as the political leader of the Rebel Alliance.
But after waging a five-year Civil War to restore the democratic ideals, the Imperials were defeated and Mothma was elected Chancellor of the New Republic.