50,000 B.C.E: Acting upon the orders from an unknown source, the Reaper's activate the Citadel and use it to teleport out from Dark Space and into the Milky Way. The Protheans are unable to fight off such an overwhelming and sudden attack, resulting in the collapse of their society and the destruction of their civilization. The Reapers spend the next several centuries systematically harvesting and exterminating the Protheans and other organic species, before destroying most traces of the Prothean Empire and retreating back into Dark Space; species that have not developed space travel are left alone and undisturbed.

While the Reapers were harvesting the space-faring civilizations in the galaxy, on the third planet orbiting a G-type star, the species known as Homo sapien gradually began to build their own society and civilization. Their ascendancy into civilization was not peaceful; conflicts occurred on an almost daily basis, and two massive wars that spanned the entire planet were fought that ravaged the entire planet. Millions of people were killed into these conflicts, and mankind edged closer and closer towards Armageddon with the advent of nuclear weapons.

Yet humanity was not solely marked by war and destruction. Science and technology thrived in human society, while scientists strove to crack and utilize the secrets of the universe itself. Two rival companies lead the way in scientific advancement: Black Mesa and Aperture Science. While the companies worked on mostly different things (Aperture researching AI's, nanotechnology, matter replicators, and genetics, and Black Mesa researching military weaponry, rocketry, experimental propulsion, and high-powered energy weapons), both companies focused on one subject in particular: teleportation. Their research into this field lead to two different outcomes: Aperture developed the Aperture Science Handheld Portal Device, a device capable of creating two linked wormholes on any flat surface. Black Mesa on the other hand discovered the border world of Xen, a pocket universe that bordered two different universes. What was of particular interest to them was the discovery of Xen crystals, crystals containing large amounts of exotic matter. Utilizing these crystals, Black Mesa was able to develop a teleportation system that "sling-shotted" an object around Xen and to a receiving teleport.

But there were greater powers at work, powers that manipulated humanity for their own plans. A mysterious entity known as the G-Man approached the administrator of Black Mesa, Wallace Breen, and offered him the purest Xen Crystal every discovered. The testing of this crystal triggered the Resonance Cascade, a catastrophic event that tore open inter-dimensional holes leading to Xen, allowing Xenian flora and fauna to flood into the facility, and allowed the being known as the Nilhilanth to send its minions to Earth. It was only through the combined efforts of Gordon Freeman, a theoretical physicist working at the facility, Adrian Shephard, a US Hazardous Environmental Containment Unit Marine, and Barney Calhoun, Black Mesa security guard and friend to Gordon Freeman that the Nilhilanth was killed and key staff members of Black Mesa saved. The G-Man soon appeared after the incident, and "recruited" Freeman and Shephard, while Calhoun was able to escape his grasp. The G-Man placed Freeman and Shephard into stasis to be awoken when they were needed. With the death of the Nihilanth, the Xenian invasion of Earth was stopped, and the Vortigaunts, previously enslaved by the Nilhilanth, were set free. They later made contact with remaining Black Mesa scientists and personnel.

While the Nilhilanth's invasion of Earth was foiled, the Resonance Cascade continued to wreak havoc on the Earth. Portal storms ravaged human cities, while depositing invasive Xenian species on the planet. Governments and the military shepherded civilians into heavily defended cities, as they waited out the portal storms.

Unfortunately, the Resonance Cascade had the unforeseen consequence of attracting the attention of the Advisors, the leaders of an all-consuming empire called the Combine that spanned hundreds of different universes. Interested in humanity's development of local teleportation technology (something the Combine never achieved), the Combine launched a massive military assault on Earth, crushing the planet's armed forces and enslaved the human race during a conflict known as the 7-Hour war. For his role in negotiating Earth's surrender, Black Mesa administrator Wallace Breen was declared Administrator of the Earth by the Combine.

During the 20 years of the Combine Occupation of Earth, the Combine ransacked the Earth, draining it of its minerals and water. Combine fleets were dispatched to the rest of the solar system, and outposts were established, as the Combine slowly drained the entire solar system of its resources. Massive 3 km high war factories and headquarters known as Citadels were teleported into every remaining city while an energy field known as the suppression field prevented humanity from reproducing; millions of people were taken away to prison-camps, where they were converted into mindless slaves and soldiers. Their efforts were opposed by the Lambda Resistance, a resistance lead by Eli Vance, Isaac Kliener, Arne Magnusson, and Barney Calhoun, and were also aided by the Vortigaunts. However, their efforts were mostly ineffective and made no difference in the Combine's Operations.

All that changed in the year 2029 C.E, 20 years after the 7-Hour war. The G-Man woke Gordon Freeman and Adrian Shephard from stasis, and tasked them with overthrowing the Combine's rule over Earth. Gordon was sent to the Combine capital of Earth, City 17, in Eastern Europe, while Adrian was sent to City 28 in North America. The two of them blazed a trail of destruction around the Earth. Freeman destroyed the main Combine Prison and augmentation facility of Nova Prospekt (inciting a world-wide rebellion coordinated and lead by the Resistance known as the Uprising), destroying the City 17 Citadel (resulting in the destruction of City 17), deactivating the suppression field, and defending a Resistance rocket that launched a dimensional buffer into orbit, preventing the formation of a Combine superportal and stopping Combine reinforcements from reaching Earth. Meanwhile, Shephard succeeded in destroying much of the Combine's spaceports and launch facilities on Earth crippling their space launch capabilities, and successfully captured a Combine anti-orbital planetary defence cannon. The use of that cannon was critical in the prevention of Combine reinforcements from the rest of the solar system landing on Earth. The result was that Combine forces on Earth and in the solar system were scattered and cut-off from all contact and reinforcements. The Resistance also too heavy losses during the Uprising; thousands of Resistance soldiers were killed, and leader Eli Vance was killed by a pair of Advisors.

Shephard and Freeman finally met face to face in Greenland, after both arriving at the site of the Borealis, a long-lost Aperture Science research vessel, rumoured to be carrying experimental teleportation technology. The two succeeded in defending the Borealis from remaining Combine soldiers and rescuing Resistance scientist Judith Mossman. With the Borealis's teleportation technology secured, Resistance leaders begin to plan for the eradication of remaining Combine forces. However, while the technology from the Borealis was interesting, the method of teleportation Aperture Science used completely baffled the Resistance, leaving the Resistance unable to use it until their scientists fully understood it.

In light of this, a more conventional battle is waged between Combine remnants and the Lambda Resistance. With Gordon Freeman, Adrian Shephard, and Barney Calhoun leading the way, the Resistance would liberate city after city, country after country, and continent after continent from the Combine, but not without suffering heavy casualties. The final assault took place against the Combine stronghold of City 37 in Australia in the year 2034 C.E. At this time, the Resistance was able to fully understand and utilize the teleportation technology found aboard the Borealis; this new form of technology was able to teleport large objects or group of objects anywhere within a range of 40,000 km, and was able to do so without having to link to another existing teleport. Using this new form of teleportation, the Resistance was able to completely bypass Combine defences strike deep within the city and annihilate Combine forces, without having to build new teleporters onsite, as had been previously done, while only suffering light casualties.

With the liberation of the Earth from the Combine, humanity and their Vortigaunts allies entered a brand new era.

2034 C.E: Gordon Freeman and Alyx Vance, daughter of the late Eli Vance, are wed in the Resistance base of White Forest. The wedding party eventually ends with the infirmary packed full of guests suffering severe hangovers.

2035 C.E: With the destruction of much of the Earth's biosphere as the result of invasive Xenian species and the Combine occupation, food is at a critical low point; current food rations can only sustain the current human population (estimated at around 2.5 billion) for at most a decade. It is the Vortigaunts, with their ancient knowledge of Antlion husbandry, that come to the rescue. The Vortigaunts are convert existing Antlion hives into massive farms, and provide much needed food, as well as producing other beneficial products, such as medicine and vortessence-enhancing agents. According to one person, Antlion meat tasted "like chicken".

2036 C.E: Massive reconstruction efforts are undertaken around the world. Cities are being rebuilt and called by their pre-occupation names, land is being reclaimed from invasive species, farms are being constructed, transportation networks are being repaired and rebuilt, and a global teleportation network is being constructed. In addition, the remaining Combine Citadels on Earth are secured by the Resistance, and the technologies inside are being studied by scientists. Christened Project Citadel, the project is predicted to bring great technological advancements to humanity and the Vortigaunts. Humanity appears to be recovering, as a massive wave of children is born and populations swell.

2039 C.E: Various reconstruction efforts are hampered by the lack of manpower. With 4.5 billion people killed or converted by the Combine, workers are stretched thin and are unable to keep various projects on date. However, this problem is soon solved with the discovery of the intact and still operational Aperture Science Enrichment Center near Lansing, Michigan. While several people were killed during the initial meeting with the overseer of the facility (an artificial intelligence composing of an sentient computer called GLaDOS and an uploaded human called Caroline), and negotiations were strained, GLaDOS eventually agrees to release Aperture Science technology and AI controlled robots to aid in the reconstruction efforts. In return, GLaDOS was promised full funding and support of the research conducted within the Enrichment Center along with test subjects (for strictly non-lethal testing) and the assurance that Black Mesa had been "completely obliterated by a thermo-nuclear detonation."

2041 C.E: Project Citadel announces a major breakthrough in their research of Combine technology. From analysis of the deactivated core and the power systems of the Citadel along with intact Combine databases, much of the underlying theory behind the Combine's manipulation of dark matter and dark energy is discovered and understood. Project scientists promise that they would soon be able to replicate a dark energy reactor, as well as reproduce many of the Combine's technology.

2048: With the help of GLaDOS and the many Aperture Science AIs, many of the reconstruction projects are nearing completion. Most cities have been rebuilt and modernized, and various pockets of dangerous Xenian wildlife are being cleared. Power stations have been built and most of the world has access to electricity; vertical and underground farms, along with Antlion ranches provide humanity's food; cars and planes begin to fill Earth's roads and skies; and the Global Teleportation Network nears completion. Project Citadel scientists are assisted by GLaDOS.

2050 C.E: The Global Teleportation Network is finally completed. The network links every major settlement, military base, research center, and transportation hub in the world, allowing the energy-efficient transportation of people, goods, cargo, etc. The network consists of two parts: small-scale teleports developed by the Resistance that teleport small objects or people, and large scale teleports developed by Aperture Science that generate two linked wormholes over long distances, allowing the instantaneous transfer of large objects or large groups of objects.

2050 C.E: Project Citadel scientists, after many setbacks and failures, finally succeed in reactivating Berlin's Citadel's Dark Energy reactor. Massive amounts of power are generated by the reactor, leading to the hope of cheap, reliable, and plentiful energy, and barring any major setbacks, production of Dark Energy reactors could begin soon.

2050 C.E: The nanoforge, a machine that can create a wide variety of non-organic materials from simple, raw materials, is developed and perfected by Aperture Science. While a limited form of the nanoforge had been in use at Aperture Science for quite some time, it was only capable of producing a limited range of materials in small quantities. By combining it with technology reverse-engineered from the Combine Citadel's factories, the new nanoforge is much more suited for wide-spread industrial usage.

2051 C.E: Medical nano-technology is distributed to the general public. The technology grants people a wide variety of abilities, such as increased healing and cognitive functions, better stamina and endurance, faster reflexes, and a longer lifespan.

2052 C.E: After 34 years of managing and governing the Earth, the Lambda Resistance is officially dissolved and a new global government called the Coalition Assembly is formed, consisting of Humans and Vortigaunts. Headquartered in New York City, the Coalition is governed by the Parliament, consisting of elected representatives, and is split into 3 branches: Legislative, Executive, and Judicial.

2054 C.E: In the light of the help and assistance given by sentient AI's, the Coalition Parliament votes in favour of the Sentient Rights Bill, which gives AI's full citizenship and rights as Coalition citizens. A new department, Research Development, is created and placed under the direct leadership of GLaDOS and headquartered in the Aperture Science Enrichment Center.

2055 C.E: With the assistance of nanoforges, AI's, and the Vortigaunts, Project Citadel scientists begin the construction of the first Coalition-made Dark Energy reactor. However, Project researchers are hampered by the lack of raw resources for the nanoforges.

2056 C.E: A global census reveals that the human population has increased by around 500 million, while the Vortigaunt population has also increased significantly. With the demand for raw resources and energy for nanoforges and other installations at critical highs, plans are drawn up for an expansion into space. That same year, using reversed engineered Combine technology and technology developed at Aperture Science, and a small colony consisting mostly of AI's are sent to Shackleton crater on the moon to mine for Helium-3 and raw-resources for nanoforges; the Helium is used in newly built nuclear fusion reactors, greatly alleviating Earth's energy problem.

2058 C.E: Project Citadel announces the completion and activation of the first Dark Energy reactor in New York. Plans are drawn up for the construction of more reactors in various cities around the world.

2059 C.E: In another breakthrough, Project Citadel scientists announce that they have synthesized the metal used by the Combine in their Citadels, utilizing a device known as the femto-manipulator. While the synthesized version of the metal is not as strong or durable as the original version, the new metal is still lighter, stronger, and more resistant than any other metal or alloy by magnitudes. The new metal is named unobtainium.

2061 C.E: Dark energy reactors and Oscillation generators are now in common use by the Coalition. The new generators, reversed engineered by Project Citadel personnel, provide far more power than even nuclear fusion reactors; when used in conjunction with teleporters, teleportation speed and efficiency is increased greatly. Research also begins in the utilization of dark energy manipulation as a form of space travel and weaponry.

2075 C.E: The Coalition now has several colonies established on the moon, and has launched a colony ship bound for Mars. Automated probes bring back tons of raw materials from the asteroid belt for the hungry nanoforges on Earth while the population of the Coalition soars. Dark energy is now in common use by the Coalition, replacing other forms of power; orbital shipyards and space elevators constructed out of unobtainium start to appear; research is conducted into the application of dark energy manipulation in faster than light travel. The Coalition enters a golden age.

2083 C.E: While digging a mine, miners from the Lowell Colony on Mars stumble upon an abandoned ancient alien bunker. The Coalition immediately dispatches military personnel to the site, and hushes up the discovery. Research and exploration of the bunker is conducted in absolute secrecy.

2084 C.E: From examination of the bunker, the Coalition announces that the ruins were constructed by an alien species called the Protheans, who had established several monitoring stations and research outposts around Earth 50,000 years ago. At the time, the whereabouts of the Protheans are unknown.

2084 C.E: Coalition scientists, while a studying the Prothean technology found within the ruins, discover an unknown element capable of manipulating dark energy fields. Upon further analysis, it is discovered that upon exposure to a negative current, the mass of an object is decreased, while when exposed to a positive current, the mass of an object is increased. However, the applications of the element, called Element Zero, are deemed to be less promising than the direct-manipulation of dark energy, and coupled with the scarcity of the resource within the solar system and its toxicity, leads to further research being abandoned.

2087 C.E: Project Citadel scientists begin to introduce dark energy weapons to the Coalition military. The weapons either convert the dark energy into a beam of exotic matter that tears apart objects, or convert it into dark energy plasma that generates tiny space-time disturbances, warping an object's structure and causing damage to it. While currently difficult to maintain and produce, it is expected that these weapons will soon see wide-spread use throughout the military.

2090 C.E: After decades of research and development, a form of faster-than-light travel is developed by Coalition scientists using a device known as a fold-drive. The fabric of space is manipulated causing the surrounding space-time to be folded, so that the space in front of it is contracted, while the space behind it is expanded, propelling the ship forward at faster-than-light speeds. The first FTL ships are sent to neighbouring star systems.

2095 C.E: Two colonies are established in the Alpha Centauri system and the Epsilon Eridani system.

2100 C.E: While exploring an anomaly on Pluto's moon Charon, field scientists accidentally activate an alien device buried underneath it. The device is a 15 kilometer long structure shaped like a tuning fork, with a core of element zero in the middle of the device; when examined by scientists, the device, dubbed a Mass Relay by scientists, appears to be part of a massive interstellar transport network, capable of transporting object over thousands of light years almost instantly. However, the Coalition's reaction at the discovery soon turns to fear and panic at the discovery that the metal used in the device is the one commonly used by the Combine, leading to beliefs that the device was constructed by the Combine. Even worse, new research of the Prothean bunker and data archives on Mars reveal that the Protheans were wiped out by another alien race 50,000 years ago; panic and rioting spreads throughout Coalition space, as rumours that the Protheans were wiped out by the Combine begin to spread.

2101 C.E: The Relay Disassembly Protocol and the Shephard Doctrine are drawn up and enacted by the Coalition Parliament. The Relay Disassembly Protocol calls for the disassembly of every Mass Relay within Coalition space, while the Shephard Doctrine states that all colonies must be heavily defended against any attack. Coalition expansion into space slows to a crawl, as the focus shifts to defense, rather than exploration.

2101 C.E: Construction of the Sol Defense Grid begins. Thousands of monitoring stations are scattered throughout the system, while massive planetary defense cannons are constructed on Earth, Mars, Venus, Mercury, and the moons of Jupiter and Saturn. Giant orbital defense platforms are built orbiting Earth; teleportation stations are built in the rest of the system, linking the rest of the solar system with Earth; military ships are constructed at breakneck speeds. The Grid will not be completed until 2134, while the Sol Defense fleet will not reach full strength until 2119. Plans are drawn up for a massive Dyson sphere surrounding the sun, for the purposes of manufacturing, defense, and habitation

For the next 1000 years, the Coalition expanded slowly, fearful of a Combine presence in the stars. The Coalition's starships, armed to the teeth with missiles, nuclear weapons, drones, star fighters, and dark energy weapons numbered in the thousands, while the 100 colonies established by the Coalition were essentially giant fortresses, heavily defended against any attack; Sol was virtually impenetrable, crawling with defense platforms, ships, anti-orbital cannons, and monitoring station, while the massive Dyson "factory" pumped out goods, munitions, and starships at an ever increasing rate. Every single Mass Relay found in Coalition space was immediately disassembled. It was only in the year 3025 that the Coalition decided to establish a new colony in a neighbouring star system. The colony's name? Shanxi.

3025 C.E: The colony of Shanxi is established. The Shanxi Defense Network begins construction immediately.

3025 C.E: Following the discovery of a relay within the Shanxi system, disassembly of the relay begins immediately.