Origins and Foundation
In 171 A.G., the Earth Kingdom had descented into chaos and anarchy following the assassination of Queen Hou-Ting at the hands of Zaheer and the Red Lotus. With her death, the nation fell apart and anarchy spread across the land. In Zaofu, Suyin Beifong was asked repeatedly to take power and stabalize the nation, but she refused, not wanting to push her ideals on the world. Kuvira, her head guard and adopted daughter, became upset with this decision and, unwilling to let her country slip back into the dark ages, decided to stabalize the continent herself. She took with her the entire guard of Zaofu, skilled fighters that wanted to make a difference, along with some of Zaofu's wealthiest citizens, Zhu Li, Varrick and Suyin's son, Baatar Jr. After proving herself by successfully stabalizing Ba Sing Se, Kuvira was trusted by world leaders and declared the de facto provisional leader of the Earth Kingdom. Kuvira and her loyalists went on to assemble an army of supporters and followers to reunfiy and stabilize the rest of the Earth Kingdom. During this time, Kuvira was succesful in ending the chaos and recruited both Bolin and many anti-royalist citizens of the Earth Kingdom.
Kuvira and her army traversed the Earth Kingdom for three years and finally ended the chaos in 174 A.G. and reunited the nation with military discipline. 90% of the country was under control and stability returned to the shattered nation but, the wealth and peace soon came to an end as Kuvira's mercy ran dry and she grew to become more domineering and tyrannical, as she began enslaving the citizens of the country to maintain her armies and those of non-Earth Kingdom ethnicity were sent to re-education facilities. Eventaully, Kuvira and her armies marched on the State of Yi and forced many bandits to swear allegience towards her and join in her chaos.
Annexation of the State of Yi
Kuvira and her forces eventually marched and arrived in the Earth Kingdom state of Yi. There, the governor of Yi was displeased with the military presence in his state and even thought that bandits overran the State of Yi itself. In a private audience, Kuvira confronted the governor and demanded that he surrender and hand over Yi to her and her forces. At first, the governor was hesitant and demanded that Kuvira leave and her army withdraw stating that he had no intention of handing over Yi to Kuvira and her troops but, she wasn't accepting his answer and instead gave him a day to reconsider. Over the next day, bandits would attack and raid supply trains and cut off any form of relief and aid to the state and so, the governor said yes to Kuvira's intervention and she returned with her army.
The Earth Empire military eventually arrived into Yi and the governor surrendered and the State of Yi was handed over to Kuvira and annexed into the Earth Empire while the bandits were finally delt with and peace was restored. Kuvira and many of her followers and officers of the Earth Empire attended an award ceremony where Wu would be crowned as the new Earth King. There, Kuvira was given an award but, she took over the event and declared the establishment of the Earth Empire, the dissolution of the Earth Kingdom and the abolishment of the monarchy leaving Wu without any form of power or authority and was met with applause from her followers. She then threatened world leaders that they would suffer military counter-attacks if they responded with force.
Battle and Fall of Zaofu
The armies of the Earth Empire continued to march forward and headed towards the state of Zaofu, which was the last Earth Kingdom state left to be annexed into the empire and its territory. During their march towards Zaofu, only a few soldiers were left behind to defend the borders of the newly formed empire with two of them having captured Ikki believing her to be a spy. After finding out that she was the daughter of Tenzin, the two ended up plotting to capture her other siblings and parents believing that Kuvira would promote them. However, the two outpost soldiers bonded with Ikki and eventually began helping her locate where Korra could've been hiding out and upon releasing her, the two soldiers were ambushed and knocked unconcious from airbending attacks by Jinora and Meelo.
Upon their arrival at the home of the Metal Clan, the army had made camp at its gates and gave Zaofu 24 hours to surrender. During this time, Bolin questioned his loyalty towards Kuvira and he eventually deserted the army along with Varrick and Zhu Li via escaping in stolen mecha suits. Their escape however, did not go unoticed and Bataar Jr. ended up leading an effort to capture the three deserters resulting in a battle that ended with the three of them being captured. Bolin was sent to a rehabilitation camp, Varrick was forced to continue developing weapons for the Earth Empire and its military and Zhu Li was pardoned after she pledged her allegiance towards Kuvira.
Attacking Republic City
After Zhu Li's assistance in constructing a weapon of mass destruction, Kuvira had it built into a secret colossus and then brought the entire army to Republic City. Since Republic City was part of a separate country, this marked the Earth Empire's first act of invading and conquering foreign territory. Kuvira invaded the city earlier than expected, but the city was able to be mostly evacuated nonetheless.
When she arrived in the city, she demonstrated her weapon's power by destroying a few structures, forcing Raiko to surrender. However, Baatar, Jr. was captured by Raiko and Korra, and he was told that he would never see Kuvira again if she didn't leave Republic City alone, agreeing that they would let Kuvira have the Earth Empire. Baatar contacted Kuvira and proposed that they settle down in Zaofu, and after confessing her love to him, she locked on to Baatar's location and fired her cannon.
Bolin's earthbending saved the group, however their mech suits that they were going to use against the colossus were destroyed in the blast. A number of tactics were used to bring down the giant mecha suit, including lavabending the suit's legs and trapping it within an electromagnetic pulse blast, but all failed. The one that works is two hummingbird suits (piloted by Varrick and Zhu Li in one and Asami and her father Hiroshi Sato in the other) drilling a hole inside the suit with plasma saws designed by Hiroshi during the Equalist revolution. Though the plan worked and Team Avatar managed to get inside the hole, it cost Hiroshi his life, ejecting his daughter from the hummingbird suit before he himself was crushed by the giant metal hand of the mecha suit.
Kuvira, now beginning to lose control of the situation, orders the suit to be put on lockdown to find the intruders. Kuvira and Korra have an intense fight, and during this fight, Mako's lightning attack on the mech's engine causes a massive explosion, splitting the colossus in half. Korra and Kuvira fall out from their half, and it seems as though the fight is over. However, Kuvira, not ready to give up, attacks Korra when she is distracted, but rather than fight her, she runs off into the Spirit Wilds. She continues to run from Korra until she finds her spirit energy cannon hanging from the vines, and uses it in a last attempt to destroy Korra. But she finds herself unable to turn the weapon off once it goes berserk and wreaks havoc on the city. Korra saves Kuvira's life by spiritbending the blast, creating a massive explosion which destroys the wilds and creates a new spirit portal.
Kuvira, now realizing the error of her ways, agrees to whatever punishment the world sees fit and surrenders peacefully, ordering her army to stand down. Three months later, Kuvira is sentenced to life in prison for her crimes, and Baatar, Jr. is sentenced to house arrest in Zaofu.
Rebuilding the Earth Empire
Little did Kuvira know, one of her best scientists, Dr. Sheng, had developed groundbreaking new brainwashing techniques, using a combination of ancient methods and new technology. However, Kuvira never was told about them because of her surrender in Republic City. Guan, the commander of the Earth Empire's southern forces, continued to pursue these techniques and refused to give up the southern Earth Empire following Kuvira's surrender.
Shortly after Kuvira was sentenced for her crimes, Guan got wind that Wu was planning on turning the monarchy structure of government in the Earth Kingdom into a democratic republic, and told his forces that he would never abandon them as Kuvira had, and that the Earth Empire would rise from defeat and become more powerful than it had ever been. Guan and Sheng continued developing their brainwashing techniques, hoping they could make the brainwashing effects permanent. After using this technology on Wu, he moved the date of the election for governor of Gaoling, which Guan won in a landslide.
Kuvira was released from prison temporarily so she could give the final order to Commander Guan's troops to surrender, they instead chanted their support to him, showing her that they were no longer loyal to her. Guan claimed that she was never fit to lead the Earth Empire, and that she was weak. Guan attempted to brainwash Korra's friends to turn against her, as well as trying to brainwash Kuvira herself. Eventually, after a fierce battle with Kuvira, Guan surrendered, but Kuvira attempted to kill him. However, Suyin stopped her, and Kuvira spared Guan's life. Dr. Sheng attempted to escape but was ultimately stopped by Asami, who stunned her with an electric glove. Guan subsequently ordered his entire army to stand down.
As of the dissolution of the Earth Empire's southern forces, the Earth Empire was permanently disbanded and the Earth Kingdom officially became a republic. Guan and Sheng's fates are left unknown, but it is likely that both of them were sentenced to prison for their attempted coup.