Earth

Earth is the third planet in the solar system, most notably inhabited by humans, and orbited by the Moon. After the Black Mesa Incident, the planet became overrun by hostile alien creatures from the Xen borderworld, and later taken over completely by the Combine.

History
Shortly after the development of teleportation technology at Black Mesa, the Xen borderworld was discovered. Scientists began investigating Xen and experimenting on the creatures found there by traveling through portals to reach it.

Experiments into mysterious crystals found on the borderworld led to a catastrophic event, dubbed the Black Mesa Incident, which tore open the gateways between the dimensions and allowed waves of aliens to spontaneously travel through and begin attacking the unprepared scientists. This resonance cascade caught the attention of the Combine empire, who, after the detainment of Gordon Freeman and the destruction of the Black Mesa Research Facility began to send their own legions to Earth. A short but decisive war between the world's militaries and the Combine ensued, later called the Seven Hour War, ultimately resulting in the capitulation of all world governments, including the United Nations, and the enslavement of Earth's human population.

During the first stages of Half-Life: Alyx it is possible to find an old-school globe model that provides various information about the state of the Earth shortly after the Seven Hour War. The model shows the North American continent crossed out with large X mark, indicating that it suffered serious devastation during the war and was subsequently abandoned with the remaining human inhabitants relocated to eastern Europe. The fate of other continents and areas of the globe, such as Asia, Africa, Oceania or Greenland, remains unknown, indicated by a question mark placed on their respective model representations.

Under Combine control, Earth was left in utter chaos. The Combine began to strip the Earth of any useful natural resources, the planet being "picked to the bone". The most noticeable sign of this was the considerable drop in sea levels as water was drained and brought to other Combine worlds. Toxic and radioactive waste is prevalent in many areas, making them inhospitable.

Deleted material from the game included a Combine airship 'absorbing' the air while distributing a collection of chemicals that are harmful to humans into the atmosphere. Also, the introduction of fierce, extradimensional lifeforms such as the Antlions and Headcrabs left possibly all of Earth's natural lifeforms extinct. Because of all of these factors, even if the Combine rule ends, extensive rebuilding, cleanup, and Xeno-extermination would have to take place to make the planet habitable again.

The originally hostile Vortigaunts united with the humans and became part of the Resistance, an underground alliance who intended to overthrow the Combine and regain control of the planet. Bases were set up and over several years they gained much information on the Combine. The reappearance of Gordon Freeman led a sudden spur of movement and attacks from the Resistance, called The Uprising, allowing them to usurp control from the Combine forces in and surrounding City 17, destroying the Citadel Core and irradiating the entire surrounding area after escaping to bases in The Wasteland.

City 17's Core destruction broke down the Combine's lines of communication, leaving them stranded within their settlements on Earth. In an attempt to bring more forces through and thwart the Resistance, the Combine induced the Citadel's destruction, causing it to produce a Superportal. At White Forest, the Resistance were able to launch a rocket into space to take advantage of a satellite network to close the Superportal before it matured and fully opened.

The Resistance has so far been successful in remaining covert and outside of the Combine's control; however, as yet the outcome of their endeavors is unknown.

However, at the end of Portal 2, it remains unclear if the Combine have won the battle. Chell emerges in a sunny wheat field, and the sky is blue and healthy-looking, unlike the polluted, dark sky the Combine take care to cultivate, and the presence of the crops seem to indicate that the Combine have failed in draining Earth's resources.

However, these elements might have just been used to enhance the idea that Chell is finally free and leaving Aperture Science. Alternatively, they may indicate that the Combine has moved the vast majority of humanity elsewhere (such as Europe) and has since concentrated its efforts there, largely ignoring North America. Although before the end of Portal 2 Chell goes through a portal onto the Moon as part of the final boss battle between her and Wheatley, and briefly looks at the Earth which appears to be healthy, with its natural sea level.