Invasion of the United Republic of Nations

The invasion of the United Republic of Nations was the final event of the formation of the Earth Empire, as it attempted to annex the territories of the United Republic of Nations to the Earth Empire, reforming the Earth Kingdom as it was before the Hundred Year War. Its primary battle took place in Republic City.

Prelude
Due to Zhu Li's desertion of the military of the Earth Empire, she was able to warn Republic City that Kuvira planned to invade the United Republic of Nations two weeks after the liberation of the Beifong family took place. As such, the city's evacuation efforts were made mandatory under the guidance of Prince Wu and Pema, the United Forces were deployed to defend the city, and Asami and Varrick increased their production lines of the hummingbird mecha suits. Meanwhile, Kuvira had her spirit energy cannon be attached to the right arm of an enormous mecha suit, which enabled her to easily transport it to anywhere she desired.

Knowing that Zhu Li had inside knowledge about her invasion plans, she executed her plans a week before scheduled. Her element of surprise was broken, however, when Team Avatar, still under the impression that the spirit energy cannon would be transported by rail lines, set out to destroy those and the weapon. Although Kuvira fired several times at them in an attempt to take them out, they managed to escape and warned everyone that Kuvira's arrival was imminent, despite the defenses of the city not being ready yet.

An unstoppable weapon
Reaching the city, Kuvira halted her army at the defense barricade erected by the United Forces. President Raiko radioed Kuvira and ordered her to leave the United Republic's territory. She refused, however, and used her spirit energy cannon to destroy the United Forces' fleet in order to demonstrate the power she possessed. Pointing her weapon to the gathered troops, she forced Raiko to surrender, lest she would kill them all. As he gave up the city, Kuvira demanded he turned over his army and the Avatar over to her, while giving Baatar Jr. his location, so he could present Raiko with their terms to consolidate the surrender. Korra refused to surrender and reunited with Team Avatar, Tenzin, Suyin, and the latter two's families.

Kidnapping Baatar Jr.
Deducing that the only way they could destroy the spirit energy cannon was to take down the mecha suit carrying it, Korra, Tenzin, Bumi, Kai, and Jinora left on a stealth mission to kidnap Baatar Jr. from his airship and after bringing him back to Future Industries, they tried to force him to disclose the schematics of the suit. Although Baatar Jr. refused to talk, despite physical threats and emotional talks from his family, he relented when Korra threatened to make sure he would never see Kuvira again unless he could convince her to leave the United Republic alone. Refusing to live his life without the woman he loved, he radioed her and pleaded with Kuvira to retreat. Kuvira had the location of the call traced, however, and although she told Baatar Jr. that she loved him and that conquering Republic City was not worth sacrificing their life together, she aimed the spirit energy cannon at his location; the blast blew up a Future Industries' factory and destroyed all the hummingbird mecha suits manufactured there, though the people managed to escape safely as Bolin used his earthbending to lift a huge piece of concrete.

Slowing down the Colossus
With Kuvira and two formations of mecha tanks crossing the Kyoshi Bridge to their location, the group split up. Asami and Varrick recovered the wounded and retreated to Future Industries Tower, where a few of the hummingbird prototypes in Asami's office could be used. Team Avatar, the Air Nation and the Beifongs headed off to slow down Kuvira's advance. Along the way, Meelo spotted a paint shop and came up with a plan. With paint strapped on, the airbenders flew to Kuvira's mecha suit and bombarded the glass to blind her of her vision. Although the metalbender had water to spray the paint off, it proved to be a distraction as Bolin dislodged the mecha suit's footing with lavabending while the Beifongs, unable to bend the suit due to its platinum armor, wrapped metal wires around the legs. The airbenders and Korra then airbent at the suit to make the mecha suit trip and fall itself. In Kuvira's attempt to break free, she fired the spirit energy cannon wildly, sweeping the city skyline and destroyed the skyscrapers. Breaking free of the wires, Kuvira regained her footing and fired at Korra and the airbenders. Having lost the initiative, the airbenders and Team Avatar retreated back to Future Industries Tower.

With the progress of the Hummingbird prototypes not going well, Varrick and Zhu Li climbed to the roof of the tower. Varrick used the tower to emit another electromagnetic blast. In a wide radius the blast disabled two waves of the Empire's mecha tanks but did not stop Kuvira's mecha suit. Baatar Jr. revealed that the suit was powered by spirit energy so it could not be stopped. At that point Lin arrived with Hiroshi Sato. He revealed that Future Industries had developed a saw that was able to cut through platinum and it can be deployed onto the Hummingbirds with some electrical tinkering despite the weight of the saw. By cutting a hole large enough for a person to slip inside Kuvira's suit, they can destroy the weapon from the inside.

The benders returned outside to buy time for the Hummingbirds to be completed. The airbenders and Korra attempted to distract Kuvira while the others used the buildings to stop the mecha suit. Kuvira again fired the cannon at the airbenders. Tenzin grabbed Jinora out of the way and were caught by Meelo and Ikki respectively. Mako, Bolin and the Beifongs toppled a building onto Kuvira's suit but it still managed to rise up. Just as the suit was about to fire on the helpless benders, the Hummingbirds (piloted by Varrick, Zhu Li and the Satos) were deployed in time to gain her attention. Varrick and Zhu Li attempted to cut a hole in the back but were forced to back away as a mecha arm attempted to swat them. Their Hummingbird was eventually shot down and the pilots forced to bail out. However they managed to lure Kuvira to a water canal, giving Korra the opportunity to waterbend at the suit and freeze it in its place. The Satos' Hummingbird then started to cut another hole on the mecha suit's right leg as Kuvira ordered more power to free its cannon arm. Hiroshi kept on cutting as the arm steadily freed itself, ignoring his daughter's warnings. The arm broke free of its icy restraints and at the last second, Hiroshi bailed Asami out of the Hummingbird. The arm swatted the Hummingbird suit with the reformed Equalist leader still inside as Asami and the others watched in horror, but the destroyed suit revealed a hole in the leg. Korra, Mako, Bolin and the Beifong sisters then rushed inside the suit.

Destroying the spirit energy cannon
Inside the mecha suit, Lin and Suyin left to destroy the cannon while Mako and Bolin head for the control room where the spirit energy was powering the suit. Korra headed upward to the command room to confront Kuvira. As Lin fought a guard, Suyin metalbent the ammunition out of rotation before Lin joined her to wreak havoc and cut off Kuvira's connection to the cannon arm. Kuvira attempted to reconnect but failed. Acknowledging the weapon is now useless, the Great Uniter took off the cannon arm, incapacitated the Beifong sisters inside the arm, and flung it and the cannon away to the Spirit Wilds. Immediately after, Korra busted her way into the command room and began dueling Kuvira in a close quarters battle.

Meanwhile Mako and Bolin arrived at the control room. After defeating the engineers inside, the brothers attempted to shut the mecha suit down but to no avail. With no other choice, Mako decided to destroy the power source with lightning despite its violent reactions if attacked unprovoked. While Bolin dragged the engineers out, Mako shot at the spirit energy source with lightning. In attempting to exit the room however, he was hit by one of the reactions and knocked unconscious, prompting Bolin to barely getting him out. The resulting explosion destroyed the mecha suit, knocking Mako and Bolin unconscious, and interrupted the duel between Korra and Kuvira in the control room.

Kuvira's last stand
In the immediate aftermath an exhausted but still walking Korra dragged the unconscious Kuvira out of the destroyed behemoth. As the metalbender woke up, Korra declared the fight to be over and demanded her surrender to Raiko, but Kuvira flung a small boulder at the Avatar before she could finish. Kuvira ran into the Spirit Wilds and Korra gave chase. The Avatar soon found Kuvira standing atop the spirit energy cannon tied up in vines but pointing its barrel at Korra. Kuvira fired on Korra who dodged the beam but the cannon inadvertently started a reaction in the wilds. The spiritual energy surrounding them began bolstering the cannon's power to uncontrollable levels, to which not even Kuvira can stop. She was eventually thrown off and targeted by the cannon its wild entanglement. However before the beam could kill Kuvira, the Avatar activated the Avatar State and shielded them both from the blast. The shield and beam collided as the reaction created a dome that covered all of downtown Republic City. Afterwards the dome shrunk to its epicenter and a ray of light shot upward from the center of the crater. Tenzin remarked that a new spirit portal had opened up in Republic City as the spirits returned from the Spirit World.

As the defenders and Earth Empire soldiers searched for Korra and Kuvira, the two emerged from the portal. Having understood each other's desires, motives, and actions, Kuvira ordered her soldiers to stand down and the battle was over. She acknowledging that in Korra saving Kuvira's life from the spirit energy cannon, the Avatar displayed such power beyond that of her own and readily gave herself up to Lin and Suyin.

Aftermath
Kuvira's arrest signaled the end of the conflict and the Earth Empire as a whole. The United Republic government and the Earth Kingdom were restored. But in light of recent events, restored Earth King Wu had decided to abolish the Earth monarchy in favor of democracy and the election of leaders in each of the kingdom's states, which had earned Korra's support.

The invasion and resulting battle that followed took a heavy toll on Republic City. Train lines had been cut to prevent civilians from evacuating the city during the battle. While much of the city had been damaged to varying degrees in the fight against the giant mecha suit, the downtown area and spirit wilds were completely annihilated, with a new spirit portal emerging from its center. Rather than rebuild the downtown areas, Tenzin and President Raiko agreed to expand Republic City beyond its current limits.