The United Powers League was renamed to the "United Earth Directorate" after the discovery of the protoss and the zerg in December 2499, and brought many dissident countries into its fold, thus expanding its sphere of influence. The UED took a more proactive, militaristic stance in regards to alien affairs and bent all of its considerable resources towards studying the aliens and their maneuverings. They spent the next several months monitoring their activity, gathering data on their strengths and weaknesses. Believing that they were capable of countering any protoss or zerg attacks, the UED formed a military expeditionary force to put an end to the conflict in the Koprulu sector by any means necessary 
The ruling council received word that a new Overmind was growing on Char and decided unanimously to send Admiral Gerard DuGalle to enslave the Overmind and use it to overthrow the Terran Dominion and pacify protoss activity in the sector. However, DuGalle was uncomfortable with the prospect of 'using' the zerg to achieve victory, feeling that they represented an unnecessary risk. He would therefore employ more conventional tactics in taking control of the Dominion. The UED justified this act as a means of assuring the survival and prosperity of humanity throughout the galaxy.
The fleet journeyed to the Koprulu sector with its troops in cryogenic hibernation. Early on, a colony was sacrificed to the zerg so that the UED could observe live zerg in action. Viewing Braxis as a vital step toward dominating the sector, the UED surrounded it with an orbital blockade, and came into conflict with protoss sent Braxis to retrieve the Uraj crystal. Vice Admiral Alexei Stukov had demanded for the protoss to surrender. PraetorArtanis led an assault force against the blockade and crippled it, allowing the protoss fleet to slip through unscathed and evade the UED's pursuit.
Undeterred by their failure to stop the protoss, the UED assaulted Boralis. Progress was slow until UED forces made contact with Lieutenant Samir Duran of the Confederate Resistance Forces, who became the first colonial conscripts of the UED and aided them in taking control of Boralis.
Soon, the UED had access to the Dominion's primary datanet and moved on to the Dylarian shipyards. Striking swiftly, UED soldiers commandeered the bulk of the Dominion's battlecruisers and used them to defeat a Dominion reinforcement fleet under General Edmund Duke. The UED then conscripted the remnants of the Dylarian defenders.
The UED found the psi disrupter on Tarsonis, and Duran convinced DuGalle to have it destroyed. DuGalle charged Duran with finding the disrupter and securing it. Stukov sent in ghosts to disassemble the disrupter, and they arrived just before Duran could plant the explosives on it. They relieved Duran and secretly reconstructed it on Braxis.
The UED tracked Mengsk and Raynor to Aiur, where they destroyed Raynor's command center. However, the zerg intervened, forcing them to retreat while Mengsk and Raynor escaped through the warp gate.
Stukov took control of a large contingent of troops and returned to Braxis to activate the psi disrupter, but Duran killed Stukov and set the disrupter's power core to overload, but UED troops reached the manual override terminal and shut the reactor core down. DuGalle ordered the disrupter kept on full power around the clock and left a garrison to protect it as well.
With the disrupter's signal hampering the zerg's communications to one another, the swarms over Char to scattered in disarray, allowing the UED to penetrate to Char's surface and overcome the Overmind's defenders. Once the UED troops had reached the Overmind, a team of specially trained medics administered powerful neurostims to pacify it.
The disrupter was destroyed by the zerg under Sarah Kerrigan with help from Raynor's Raiders and a psi emitter loaned by Mengsk. The UED sent enslaved zerg to reinforce their troops on Korhal. However, their control of the Overmind was tenuous, so not many were sent. The UED forces occupying Augustgrad were overwhelmed by Kerrigan's zerg.
Two days after Kerrigan returned to Tarsonis to rest, the UED sent hundreds of zerg to Tarsonis, which was too remote for them to maintain direct control, so they sent thirty scientists to coordinate their attack from a small facility towards the rear of their landing zone. The attack failed when the scientists were killed.
The UED forces on Char were defeated by Kerrigan's zerg and DarkPrelateZeratul. The remnants of the UED fleet fled Char, then attacked Char Aleph alongside Mengsk's fleet and a protoss armada under PraetorArtanis. DuGalle offered terms of surrender when it became clear that he could not hope to overcome Kerrigan at her full strength. The UED fleet was allowed to flee in the direction of Earth with a reasonable head start. The fleet was soon overtaken by Kerrigan's forces and eradicated. No UED ship made it back to Earth to report the events that had taken place in the Koprulu sector.
The UED attempted to discover what had happened in the Koprulu sector, but their spy network returned only incomplete information.
In the following years, the UED turned its attention to internal affairs.
Interest in the Koprulu sector waned but it was not forgotten. The defeat was a temporary setback; the loss of the expeditionary fleet had only a minor impact on UED strength. The UED needed time to plan and assemble political support for a future invasion. The UED's return was expected in the sector. As late as 2505, the population of the Terran Dominion still had the suspicion that Earth's government was still out there, observing them.
The UED Expeditionary Fleet used nearly identical technology and munitions as the Koprulu Sector terrans. Notable exceptions included infantry small arms.
The terrans of the Koprulu sector developed technology based on ATLAS's databanks after the Long Sleep. As the computer originated on Earth, the two cultures would have a common technological base with similar characteristics. However, it would be more likely for the UED to be more advanced. Chris Metzen speculates that another reason the UED and the Colonials used similar equipment is because the UED left their "big guns" behind and relied heavily on local production and technology, such as the Dylarian Shipyards, the Confederate Resistance Forces of Samir Duran, and the Psi Disrupter. That would also allow the initial UED invasion force to be smaller.
It appears that the UED may use a different calendar than its terran counterparts, as the year 2500 has been displayed as being in the year 872, making Year 0 1629 AD. However, with the lack of any reference to the nature of this date having been made and that '99 has also been used to depict dates (coinciding with the year 2499), this may be a cinematic error. Heroes of the Storm indicates that the Gregorian calendar is still in use by the people of Earth.
Chris Metzen speculated about the nature and level of UED weapons, making an aside about "uber tachyon lasers" and other "big guns" remaining behind in UED territory proper during the Brood War.
The United Earth Directorate appears to be loosely based on the United Citizen Federation of the movie Starship Troopers. Similarities can be seen at the end of Episode V, during the UED Victory movie.
The orientation of the Earth in the UED symbol changes multiple times during its appearance in the UED Victory Report cinematic, with some appearances showing an Atlantic-centered orientation while others show a Pacific-centered one. In addition, the size of the continents changes between flag appearances, most notably with the African continent.
Heroes of the Storm presents a different account of the UED's motivations for invading the Koprulu sector, claiming that the terrans of the sector's motivations had wavered over time, and that they were a threat to the UED's own sphere of influence. This contradicts the lore of both StarCraft and its expansion, which established that the terrans of the sector never had any contact with Earth, and that it was the discovery of alien species that was the impetus for the UED's formation.
↑Samir Duran: "Captain, I am Lieutenant Samir Duran of the Confederate Resistance Forces. I've been monitoring your attack against the Dominion, and I'd like to offer you my services and skills in return for amnesty." Alexei Stukov: "Intriguing. Lieutenant, you have absolutely no idea as to who we are or what we have come here to do. Why do you wish to side with us?" Duran: "Because my men and I have sworn to fight against the Dominion and its Emperor. Seeing as how your forces have been laying into this Dominion base, I figured we were on the same side." Stukov: "I see. How exactly do you propose to be of service to us, Lieutenant?" Duran: "Well, I possess intimate knowledge of the surrounding area. I can show you an alternate route that leads to the rear of the main Dominion base." Stukov: "Very well, Lieutenant. Consider yourself and your men the first colonial conscripts of the United Earth Directorate. Captain, see that the Lieutenant is given our full cooperation. We shall see if he's worth the effort." StarCraft: Brood War. Vivendi Games. Level/area: First Strike (in English). 1998.
↑Adjutant: "We have impressed most of the remaining Dylarian defense forces into the fleet and stand ready to move to the next phase of our invasion." StarCraft: Brood War. Vivendi Games. Level/area: Ruins of Tarsonis (in English). 1998.
↑Duran: "I must humbly suggest to you, Admiral, that the Disrupter be destroyed as quickly as possible. If it should fall into the hands of the Dominion we could face some very serious problems." Gerard DuGalle: "Hmm. A prudent call, Lieutenant. I am inclined to agree." Stukov: "But Admiral, this device could insure our victory over the Zerg! It is foolhardy to discard such an advantage!" Duran: "Surely, Vice Admiral, you don't doubt the tenacity of your own fleet? Your Directorate forces will devastate the Zerg without the need for such a device." Stukov: "Listen here, Lieutenant, I've had just about enough of your-" DuGalle: "This conversation is over, Alexei. Given the true nature of our mission to enslave the Overmind, we could hardly stand to have Mengsk control a device of this magnitude. This Psi Disrupter must be destroyed in order to safeguard our mission. Ensign, set your course for Tarsonis." StarCraft: Brood War. Vivendi Games. Level/area: Ruins of Tarsonis (in English). 1998.
↑Adjutant: "Admiral, our sensors have detected a fully functional Psi Disrupter on the planet's surface directly below our position. It would appear that the Vice Admiral did not destroy the Psi Disrupter on Tarsonis as was ordered." StarCraft: Brood War. Vivendi Games. Level/area: Patriot's Blood (in English). 1998.
↑Duran: "Vice Admiral Stukov, I am here under orders to terminate your command." Stukov: "Lieutenant Duran. I'm not surprised. We both know exactly what it is that you're here to terminate. Get on with it." Duran: "Say goodnight, Stukov." Stukov: "To hell with y- !" DuGalle: "Lieutenant Duran, report your situation immediately!" Stukov: "Gerard... old friend. You do indeed have a traitor in your midst... but it's not me. Duran. He has been playing us both from the start... He convinced you to destroy the Psi Disrupter... even though it was our best chance at defeating the Zerg. And then, on Aiur, ...he allowed the Zerg to overrun us even as we had the fugitives in our grasp. I came here... to activate the one hope we have of defeating them... And you killed me for it. Heh, heh... Duran is your enemy Gerard. I suspect... that he may even be infested as well... Use the Disrupter, Gerard... Finish our operation... Let my death have at least some meaning." DuGalle: "Alexei! No! What have I done? What have I done? Computer, where the hell is Lieutenant Duran?" Adjutant: "The Lieutenant no longer registers on my sensors, Admiral. He appears to have vanished." DuGalle: "No." StarCraft: Brood War. Vivendi Games. Level/area: Patriot's Blood (in English). 1998.
↑DuGalle: Once you have reached the Overmind and crushed its defenders, you are to send in specially trained medics who will pacify the creature with potent Neurostim drugs. Only then will our victory be assured. StarCraft: Brood War. Vivendi Games. Level/area: To Chain the Beast (in English). 1998.
↑Duran: "My Queen, we have located a small Terran facility towards the rear of the Renegades' landing zone. There appears to be a number of UED scientists working within the compound, but we are unable to determine the purpose of their presence..." Sarah Kerrigan: "I know what they're up to. I suspected that the UED's control of the Overmind was tenuous. Tarsonis is too remote for them to maintain direct control over the Swarms, so they've sent these 'scientists' to coordinate their attack." Duran: "Cerebrate, focus your attacks against the Terran facility. All of the UED scientists must be killed. Only then will we be able to finish off these Renegade Zerg." StarCraft: Brood War. Vivendi Games. Level/area: Fury of the Swarm (in English). 1998.
↑DuGalle: "Kerrigan, I am prepared to offer terms of surrender. It is my wish that you allow my men to live-" Kerrigan: "You're not in a position to offer me anything, Admiral. And I don't take prisoners. I'll tell you what... Turn your remaining ships around and start heading back towards the Earth. I'll let you have a head start, before I send the Swarms after you. It'll be interesting sport to see how far you get before you die." StarCraft: Brood War. Vivendi Games. Level/area: Omega (in English). 1998.