Earth

"Blue sky, one sun. Oh no. This is Earth, isn't it?"

- Thor upon arriving to Earth

Earth is the third planet from the Sun in the Solar System, home to the humans, Inhumans, and a number of Asgardian immigrants, and is one of the Nine Realms aligned by the cosmic nimbus, Yggdrasil. The various races of the Nine Realms refer to Earth as "Midgard", while the alien races living beyond the realms call it "Terra".

Kree Interference
"What did the Kree want?" "Change."

- Skye and Raina

In a time of unrecorded history, even before the pyramids, Earth was visited by the extraterrestrial and technologically advanced race known as the Kree. Hoping to win their prolonged war with the Nova Empire, a radical Kree faction experimented on some of the humans that they had encountered, dramatically altering their genetic structures with hopes of creating powerful bio-weapons. These modified humans, who would come to be known as Inhumans, were granted the potential to obtain superhuman characteristics if exposed to the mutagenic mist exhaled from the Terrigen Crystals. When this Kree faction dispersed from Earth, they left behind two constructs that would ensure the final transformations of the Inhumans: a city buried deep beneath the surface of the planet; and the Diviners, devices that contained Terrigen Crystals and could only be physically handled by Inhumans. Once the Diviners were joined with a temple-like structure built within the center of the city, they would unveil the crystals inside and trigger the transformation brought on by the mist.

The Convergence
Earth is one of the Nine Realms, as a result of which it is known to have experienced a cosmic phenomenon known as the Convergence every 5,000 years by which the spatial boundaries between the worlds, on certain locations on the planet, would cease to exist due to the appearance of wormholes. The planet had experienced several of these events which caused ancient civilizations and tribes to construct objects such as the Stonehenge to mark the location of the event.

The Dark Ages
"Once, mankind accepted a simple truth: that they were not alone in this universe. Some worlds man believed home to their gods. Others they knew to fear. From a realm of cold and darkness came the Frost Giants, threatening to plunge the mortal world into a new ice age. But humanity would not face this threat alone."

- Odin

Jotuns Raid the Nine Realms
In 965 AD, a war between two Nine Realms' races, the Asgardians and the Frost Giants, made its way to Earth, with the Asgardians defending the humans from the Frost Giants. The humans, in particular the peoples of Northern Europe, worshiped the Asgardians as deities, and the Asgardians would often visit Earth and teach the humans language and culture. Early Norse cultures learned much from these visits, although in time the Asgardians would withdraw back to Asgard and their memory would fall into myths and legend over the generations.

Asgardians' Descend on Earth
In the 12th century, an army of Berserkers came to Earth from Asgard. When they returned home, they left behind an Asgardian soldier with the Berserker Staff, the source of their power.

The Dark One
Since their creation by the Kree, the Inhumans continued to flourish, producing more of themselves with various new powers and capabilities emerging. However, one particularly powerful Inhuman began abusing his power and became a malevolent, god-like being whose power slowly provided him with a cult of fanatical followers that would eventually evolve into the terrorist organization known as HYDRA. Fearing his power, other Inhumans of that era utilized a Kree teleportation device known as the Monolith to transport the dark Inhuman to the planet Maveth. HYDRA dedicated the next few centuries to weakening and subjugating the world powers in preparation for their leader while simultaneously working to try and find a way to bring him back.

World War II
"I come from Earth, a planet of outlaws."

- Star-Lord

The 1940's were a tumultuous time. Depression years and racial disharmony in Europe sparked World War II, pressing various factions into a technology race. While Nazi forces occupied much of Central Europe, a detachment calling themselves HYDRA were making gains in unconventional warfare. Their leader, Johann Schmidt, actually managed to procure the Tesseract, an Infinity Stone that the Asgardian king Odin left on Earth some time after the Frost Giant attack in Norway. In studying the Tesseract, HYDRA was able to devise new and deadly weapons for their war effort. In America, Abraham Erskine and Howard Stark were poised on the verge of breakthroughs that would set the stage for the world of the modern "superhero". Erskine had developed a "super soldier" serum that was to transform an ordinary man into something altogether stronger, faster, and more resilient. As a result of this experimental process, Steve Rogers, an ill-fitted man from New York City, became the world's first superhero, "Captain America", but not without cost. Erskine was assassinated by HYDRA operatives and what remained of his serum was destroyed before the project could be pushed into widespread use. In spite of this, Rogers proved to be a brilliant military expert and soldier and led many soldiers into victory, eventually bringing down HYDRA and defeating Schmidt. However, a final mission against HYDRA left Rogers stranded in the Arctic, where he was left frozen for the next seven decades. In the end, the war was won, not with super soldiers, but with many countless lives and simple, bloody warfare. The Strategic Scientific Reserve, founded by Howard Stark as a counterpart to HYDRA, continued operation after the end of the war. Peggy Carter, Rogers' love interest, was a member though she was treated as less than equal by her co-workers, the other agents. After Stark was accused of treason and selling weapons to America's enemies, Stark sought her help in clearing his name and left her his butler Edwin Jarvis as a companion and confidante. Actively working against the SSR, Carter went on many missions assisted only by Jarvis to attempt to solve the Stark case before her co-workers did. Simultaneously, Carter and the SSR worked against Leviathan, a secret Russian organization that slowly emerged against the U.S. After the situation was sorted out, two of the SSR's prisoners Arnim Zola and Johann Fennhoff met in a cell and Zola recruited Fennhoff into his plan to rebuild HYDRA. With Operation Paperclip, the United States Government including the SSR recruited former Nazi scientists, Zola included, to work for them, and so S.H.I.E.L.D. was created.

Formation of S.H.I.E.L.D.
Howard Stark, who had assisted with Erskine's program, went on to develop numerous advances in technology. He accomplished much over the next several years, assisting in the founding of the world-spanning security agency, the Strategic Homeland Intervention Enforcement Logistics Division (S.H.I.E.L.D.). Learning from the recovered Tesseract, he went on to develop prototype Arc Reactors, powerful sources of energy in their own right. He eventually died in a car accident, leaving his business empire to his young son, Tony, and his company, Stark Industries, grew into a world renowned supplier of weapons technology. While S.H.I.E.L.D. continued to act as it saw fit, no one realized that HYDRA, still kicking yet weakened after the war, had secretly infiltrated the agency in their mission to destabilize global order and bring back the dark Inhuman.

Visitations
Extraterrestrials continued to visit the planet, but on individual missions, unlike the Frost Giants or the Kree. One such being had a relationship with Missouri native Meredith Quill and fathered a son named Peter; years later, he tasked the interstellar criminal group known as the Ravagers to retrieve the boy. The child's unexpected departure into outer space instigated his eventual role as the leader of the intergalactic justice squad, the Guardians of the Galaxy, who would later save the planet Xandar from Ronan the Accuser.

The Age of Heroes
"If we can't protect the Earth, you can be damn sure we'll avenge it!"

- Iron Man to Loki

The dawn of the superhero age might have looked like a brief spark to those who grew up reading the exploits of Captain America, but it was soon to flare into a far brighter flame. The secrets of Erskine's formula had baffled scientists for decades until Doctor Bruce Banner, a graduate from Culver University, explored a new direction and bombarded himself with Gamma Radiation to prove his theories. His experiment was successful in augmenting the human form, but the extent of the mutation was far beyond anything anyone could have anticipated. Banner was transformed into a raging green beast that was later dubbed the "Hulk", and found himself being hunted by military operatives that sought to exploit the transformation for the army's weapons division. The Hulk, however, would prove to have the heart of a hero by stopping maddened soldier Emil Blonsky, who used a similar variation of Banner's formula to transform himself into a grotesque, Hulk-like creature that rampaged through Harlem.

In response to threats posed by such things as the Hulk and the Tesseract, governments of the world formed the World Security Council, which in turn channeled resources into S.H.I.E.L.D., appointing Nick Fury as the agency's director. Idealistic himself, Fury saw more potential in superhumans than the council did, and spent the next several years putting together a list of dossiers regarding certain individuals with superhuman abilities. This list would later be called the Avengers Initiative. Tony Stark, who had spent the years since his father's death learning the role of businessman and further developing his father's company, became a pivotal participant in the superhero age. An inventive genius himself, he was personally responsible for many of the innovations that made Stark Industries a world leader in weapons technology. In 2010, while on a trip to Afghanistan, Stark's convoy was ambushed by the Ten Rings terrorist group and he was held captive and forced to develop weapons for the organization. He escaped from his captors by building a battlesuit for himself rather than the missile system they wanted, but the whole incident had a profound effect on him. He turned his company away from the path of violence, while constructing a series of high-tech battlesuits that he hoped to use to achieve world peace. Stark's vigilante actions had the effect of ushering in a new superhero age, and taking a leaf from the press, he began publicly referring to his heroic persona as "Iron Man".

S.H.I.E.L.D. soon took an interest in Iron Man's activities. Nick Fury approached Stark on several occasions and reviewed him for potential inclusion into the Avengers Initiative, but ultimately withdrew from seeking his personal involvement due to his erratic nature. Instead, S.H.I.E.L.D. turned their attentions to exploring Stark's weapons technologies. Their mandate was to protect the world from such powerful threats as the Hulk, and to this end they began to grow from a clandestine police agency into a powerful military force.

A Changed World
"We knew, that despite all diplomacy and the handshaking and the rhetoric, to build a better world sometimes means having to tear the old one down. And that makes enemies."

- Alexander Pierce to the Captain America

The most significant development in human history took place in the summer of 2011, when the Asgardians, thought to be nothing more than a myth after centuries of absence, returned once again to the world when Thor, the heir to the throne of Asgard, found himself banished to Earth after unintentionally brewing another war with the Frost Giants. While his initial appearance went unnoticed to all but a few scientists, his hammer Mjølnir caused quite a stir after it impacted in Puente Antiguo, New Mexico, and also drew the attention of S.H.I.E.L.D. After numerous altercations, Thor's adopted brother, Loki, sent the Asgardian Destroyer to Puente Antiguo to kill Thor and ensure Loki's own coronation. However, Thor reclaimed Mjølnir and defeated the Destroyer, and announced that S.H.I.E.L.D. could thereafter count him as an ally. This event revealed to humanity that they were not alone in the universe, and the now paranoid World Security Council withdrew all of S.H.I.E.L.D.'s resources away from the Avengers Initiative, and more on developing weapons to deter possible alien threats. Later that same year, a S.H.I.E.L.D. expedition team found Steve Rogers' frozen body in Arctic, and due to the effects of the super soldier serum in his body, he was still alive and revived. Having nowhere else to go, Rogers joined S.H.I.E.L.D.'s services and became a superhero once again.

The War for Earth
"I remember you tossing me into an abyss. I who was and should be king!" "So you take the world I love as recompense for your imagined slights? No, the Earth is under my protection, Loki."

- Loki and Thor

By 2012, the Avengers Initiative had been dropped on the council's orders, but was soon brought into effect when Loki, having being banished from Asgard after the events in Puente Antiguo, made a surprise appearance and stole the Tesseract away from S.H.I.E.L.D. in a campaign to conquer Earth. In a desperate bid to counter this threat, S.H.I.E.L.D. reactivated the Avengers Initiative, calling in Tony Stark, Bruce Banner, and Steve Rogers. Thor and elite S.H.I.E.L.D. agents Natasha Romanoff and Clint Barton also provided support, and together, the six individuals formed the first official Avengers lineup. The team battled Loki and his allies, an alien race called the Chitauri, in New York City, and eventually defeated them both. The Avengers Initiative was reactivated in light of its success, and S.H.I.E.L.D. began seeking out other superhumans. Phil Coulson, a high-ranking agent, was assigned leadership over an elite team of operatives that specialized in seeking out such individuals. Over the next few years, the Avengers would continue to act independently, fighting off numerous threats that S.H.I.E.L.D. could not. Tony Stark, suffering from severe post traumatic stress disorder following his experience with the Chitauri, came into conflict with an old enemy of his, Aldrich Killian. Killian attempted to control both sides of the War on Terror and monopolize from the war through use of the Extremis formula he created. With help from his two close friends, Pepper Potts and James Rhodes, Stark defeated Killian, but pockets of Extremis were recovered by agents of the Centipede Project, an off-shoot of HYDRA, who then came into conflict with S.H.I.E.L.D., more specifically Coulson's team. In 2013, the Convergence occurred in London, and Malekith, the leader of a hostile race called the Dark Elves, tried to use it as a means of unleashing the destructive Aether upon the Nine Realms. Malekith nearly succeeded in annihilating all of the Nine Realms, including Earth, but was defeated by Thor. Pieces of Dark Elf technology left behind after the battle were recovered by S.H.I.E.L.D.

The Fall of S.H.I.E.L.D.
In 2014, S.H.I.E.L.D., growing increasingly paranoid over the growing supernatural phenomena, took a greater step in its development. Through an operation dubbed "Project Insight", S.H.I.E.L.D. hoped to use three advanced Helicarriers to preemptively eliminate potential threats. However, the entire operation backfired on them when the HYDRA infiltration, having remained low for the past decades, emerged through corrupt senior S.H.I.E.L.D. operative Alexander Pierce. Pierce attempted to use Project Insight to bombard the East Coast and impose a new world order, but through the combined efforts of Captain America, Falcon, Black Widow, Maria Hill, and Nick Fury, Pierce was thwarted.

The Inhumans and Ultron
Unfortunately, HYDRA got the last laugh, as their uprising left S.H.I.E.L.D. broken, forcing the lawful good operatives to work independently. Phil Coulson's team continued to work to bring down any HYDRA agents that were not already taken down. Eventually, Nick Fury tasked Coulson with rebuilding S.H.I.E.L.D. as its new director. Under Coulson's control, the new S.H.I.E.L.D., while few in number, were able to greatly hinder HYDRA's attempts at regaining control, regaining trust from various Earth governments in the process. Their battle against the HYDRA forces under the control of Daniel Whitehall also unveiled the existence of the Inhumans to modern humanity. S.H.I.E.L.D. initially desired to learn more about the species peacefully, but their leader, Jiaying, had developed a psychopathic hatred towards mankind due in part to a horrific experience with HYDRA that left her nearly massacred. Jiaying sought to start a war with humanity, but it was quelled with the help of Skye, an Inhuman S.H.I.E.L.D. agent and Jiaying's daughter. However, during the war, artificially created Terrigen Crystals fell into the ocean, where they would ultimately be ingested by the sea life and turned into fish oil pills sold in stores. Consumption of the pills caused a global Terrigenesis outbreak, with new Inhumans emerging and prompting the United States Government to create the Advanced Threat Containment Unit to hunt down and persecute them. Their activities led to multiple conflicts with both S.H.I.E.L.D. and various transmutated Inhumans. S.H.I.E.L.D.'s efforts in bringing HYDRA down was met with seemingly charitable results, but would prove to cause even worse unforeseen events. In 2015, S.H.I.E.L.D., using information recovered from HYDRA, had the Avengers reassembled to take back the alien scepter that Loki used in his campaign against the human race; HYDRA leader Wolfgang von Strucker was conducting experiments with the weapon and the Infinity Stone contained within. The only successful subjects were the twins Pietro and Wanda Maximoff, who developed extraordinary powers. The Avengers succeeded in recovering the scepter and destroying Strucker's HYDRA cell, but Tony Stark's meddling with the device and the Infinity Stone resulted in the creation of Ultron, an artificial intelligence conceived by Stark as being a global peacekeeping program designed to replace the Avengers. Moments after his activation, Ultron went rogue and immediately started up a mad plan to achieve world peace via humanity's extinction, and recruited the Maximoff twins to assist him. Together, the trio battled the Avengers numerous times, but the Maximoffs eventually found out about Ultron's genocidal intents and rebelled against him. Teaming up with the Maximoffs and the benevolent synthetic, the Vision, the Avengers fought Ultron a final time in Sokovia. While the Avengers were victorious, public opinion against them was strained as a result of the collateral damage and death created by the conflict, and the team became fragmented, resulting in a new lineup.

A Smaller Scale
While the Avengers and S.H.I.E.L.D. continued to fight threats on a global level, other heroes emerged who fought for justice on a smaller scale. One such group of heroes was the Defenders, a foursome of street-based vigilantes dedicated to using their powers to make their respective sections of New York City safer places to inhabit. The members of the team, which included Matt Murdock, Jessica Jones, Luke Cage, and Danny Rand, squared off against various criminal masterminds such as Wilson Fisk and Kilgrave. When corrupt businessman Darren Cross forged a partnership with HYDRA to put the Yellowjacket Suit into mass production, Hank Pym sought out small-time criminal Scott Lang. Long-retired from his role as Ant-Man and unable to take action himself, Pym chose Lang to be his successor. With Pym's guidance, Lang stopped Cross' dealings with HYDRA and defeated the madman. Though much less extensive on a global scale, Lang's activities did attract the attention of the Avengers, who sought to recruit him into their ranks.

Trivia

 * Because of the numerous alternate universes of Marvel Comics, this Earth from the Marvel Cinematic Universe is officially known as Earth-199999.