From Halopedia, the Halo wiki
Earth's national governments had established joint efforts to colonize the moons, along with several other bodies in the Sol system, by 2080. In the mid-22nd century, before the formation of the UNSC, unrest began brewing on the moons as various Unified German Republic corporations based there became targets for attacks perpetrated by neo-communist Koslovics. As a result of an increasing anti-Koslovic sentiment, a resurgent fascist movement known as the Frieden arose on the Jovian Moons. Supported by Unified German Republic corporations, the Frieden opposed both the Koslovics and the United Nations' influence, seeking to achieve peace by destroying the "oppressors on Terra Firma".
In 2160, the Frieden attacked the UN Colonial Advisers on Io, sparking a three-month conflict known as the Jovian Moons Campaign, followed by more conflicts across the Sol system, including the Rain Forest Wars, a series of clashes of Mars, and the Interplanetary War. The Frieden and the Koslovics were eventually defeated, and the moons were transferred to the control of the United Nations.
Moments before the Battle of Earth during the Human-Covenant War, the Prophet of Regret's Fleet of Sacred Consecration was detected near Io by Io Station. Numerous habitats in the Jovian Moons survived the Covenant assault largely unscathed.
After the New Phoenix Incident in July 2557, the former captain of the UNSC Infinity, Andrew Del Rio, utilized his connections and accumulated favors to begin an obscure, though rewarding, political career on one of the Jovian moons and eventually replaced a deceased senator of the Unified Earth Government Senate.
The Galilean moons
The fourth largest, yet only fully terraformed Jovian moon. It features a frosty atmosphere. Although scientists had theorized that it might already contain oceans beneath the surface with amino acids and the building blocks of life, the first colonists found that this was not the case. Colonization occurred without the discovery of any new life forms.
The third largest Jovian moon. This is where the Frieden first attacked the United Nations. Despite being massively volcanic and atmospherically volatile, Io supports a large colony of humans. The fleet of the High Prophet of Regret passed near Io where they were detected by Io Station, just minutes before the Battle of Earth, allowing UNSC forces to muster a timely response.
The largest Jovian Moon and in the Sol System, bigger than both Luna and Mercury. It has been partially terraformed, though supposedly retains a harsh climate. Ganymede is well-populated. The MJOLNIR EVA variant was developed here.
Callisto is the second largest Jovian Moon. In 2170 it was the site of the signing of the Callisto Treaty which officially ended the Interplanetary War. With little civilian population to speak of, Callisto's purpose is primarily militaristic in nature; the moon's several bases are capable of launching their units and ships within an hour's notice. Herzog claimed to have to deal with paperwork regarding surveillance here.
"Jovian" is the adjective form of "Jove", who was the god representing Jupiter in the sky.
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