Under orders from Treize, Lady Une planted a bomb and used it to assassinate Vice-Foreign Minister Darlian, a politician within UESA who sought peace with the colonies. She then attempted to kill his adopted daughter, Relena Darlian, as well as she was a witness to the crime, but she was stopped by Treize.
After Treize manipulates Gundam pilot Heero Yuy into attacking a UESA conference and killing the moderate politicians in it, Lady Une assists him performing a military coup against the UESA government. The two have General Septum, the only survivor of the conference, deliver a televised speech declaring war on the colonies before Une shoves him out an airplane.
Later, Treize and Une are pursued by Gundam pilot Chang Wufei, who attempts to get revenge on Treize for his manipulations of Heero. Une ambushes Wufai in a modified Leo, but is beaten by the power of Wufai's Gundam Shenlong. Une later watches as Treize defeats Wufai in a duel but spares his life. She asks Treize why he allowed Wufai to live, to which Treize answered that he wished to have a rematch with Wufai in mobile suits.
Following OZ's coup against the UESA military, Une was put in charge of OZ's space and anti-Gundam task force. As the Gundam pilots continued to oppose OZ's domination of the Earth, Une and her forces pursued them and eventually cornered them in Siberia. To get them to surrender, Une aimed OZ's orbital satellites at their home colonies and threatened to fire unless they laid down their arms. However, Heero Yuy then attempted a suicide detonation of his mobile suit, buying time for the other Gundam pilots to get away.
Une later attended a conference with Treize, where she was almost killed by a vengeful Relena. Une and her soldiers attempted to capture Relena, but she narrowly evaded them. Due to this and her other recent failures against the Gundam pilots, Treize scolded Une and reassigned her to serve as OZ's ambassador to the Sanc Kingdom as punishment, telling her to be more ladylike. Unable to endure Treize's criticisms, Une's personality split in two with a second, more kinder and graceful personality manifesting within Une's mind known as "Saint Une". Saint Une was much more ladylike than Une's other, original personality, "Iron Une", and believed in peace with the colonies. Saint Une made several passionate speeches promoting pacifism, improving OZ's reputation among the colonists.
Shortly after OZ's backers, the Romefeller Foundation, removed Treize from his position as chief of OZ, Chief Engineer Tsubarov of Romefeller staged a coup against Une in the colonies, shooting her in the end and causing her to fall in a coma.
While she was comatose, Une's two split personalities, Iron and Saint Une, reconciled back into one personality, and when Une awoke she gained both the pacifistic thoughtfulness of Saint Une and the military prowess of Iron Une. Une soon discovered that, while she was comatose, Zechs Merquise had gone on to lead the White Fang and was now fighting a war in space against Treize, who had returned to his position as leader of OZ and was now the commander of all of Earth's forces. Taking Heero's abandoned Gundam, Une traveled into space to resume her service to her former leader. After saving Treize during fight with Zechs and arriving at Treize's MO-II command center, Une reassumes her role as Treize's second-in-command.
Une later observes via radio Treize's final battle against Chang Wufai. Though she seems to have been aware of Treize's intentions to allow himself to die in the battle his death still hits her hard. When Relena and others arrive at MO-II, Lady Une offers Relena a gun for her to get revenge for her father by shooting Une, but Relena refuses and states that it is time for an end to all conflict. Hearing this, Une contacts former OZ officer Lucrezia Noin and tells her to broadcast the duel between Heero Yuy and Zechs Merquise to the world and the colonies. Seeing the outcome of the battle as the war comes to an end, the people of the Earth Sphere become ready to embrace total pacifism and unify under the Earth Sphere United Nation.
After the war, Lady Une and a select number of other soldiers were allowed to keep their arms while the rest were disassembled and formed a group known as the "Preventers" to prevent wars breaking out again.
However, another armed conflict soon began when a revolt spearheaded by Mariemaia Khushrenada, the illegitimate daughter of Treize, and funded by the Barton Foundation occurs. The Mariamaia Army takes Vice-Foreign Minister Relena Darlian hostage and begins an invasion of the Earth. In response, Une approaches Zechs and offers to give him a Tallgeese III so he can help stop the Mariemaia Army. After Mariemaia's forces occupy the defenseless Earth Sphere and occupy Brussels, Belgium, where ESUN's headquarters is, Une manages to infiltrate the bunker where Mariemaia and her grandfather, Dekim Barton, are hiding out and confronts them. Upon hearing from Mariemaia that she is intending to follow her father's ideals, Une responds by telling her that she is only following ideals twisted by Dekim.
When Heero Yuy attacks the bunker in Wing Zero, Une protects Mariemaia and Relena from falling rubble, saying she cannot allow the daughter of Treize to die. As the civilians rise up against the Bartons and their forces, Une tells Mariemaia that her father respected those who could fight a losing battle, and that is why the civilians are siding with the Gundams and not the victors. However, Dekim refused to relent and attempted to have Relena executed by Mariemaia. When Mariemaia was slapped to her senses by Relena, Dekim attempted to shoot her himself but the shot was blocked by Mariemaia. As Dekim prepared to fire at Relena again, Une attempted to block the second shot but before Dekim could fire, he was killed by a soldier loyal to Treize, who apologized to Treize for betraying his ideals. Une then ordered the soldiers to take Mariemaia to a doctor.
After the war ended, Une adopted Mariemaia and raised her in honor of Treize's memory.