Interplanetary War (Earth-5875)

The Interplanetary War, also known as World War III, was a ware that occurred through the Sol system between the United Nations and its United Nations Space Command against the fascist Settlement Defense Front and the neo-communist Koslovics. The war was a culmination of multiple prior conflicts, such as the Jovian Moons Campaign, the Rainforest War, and the Mars Campaign.

Background
As of the 22th century, overpopulation and unrest mounted on Earth, with a few number of new political movements were formed, such as the Jovian Settlement Defense Front (SDF), a group of terrorist supremacists, as well as the neo-communist group known as Koslovics, led by the revolutionist and separatist Vladimir Koslov. As their first move, the SDF attacked and killed the United Nations' colonial advisers on the moon Io, as well as many other UN facilities and military forces across the Sol system. The increasing pattern of the attacks led to the Interplanetary War in 2164.

The War
In response to these attacks, the United Nations used their increasing influence on Earth to bring its national governments under it's power as a buildup, forming the United Nations Space Command, comprised of soldiers assembled from the many armed forces of most of Earth's countries. Following the succesful first deployment of UNSC Marines against SDF forces on Mars, recruitment and propaganda became widespread and dictated the tactic that would be used by the UN through the conflict. The Marine assault on Mars marked the beginning of the war, with the UN and the UNSC dedicated resources to crush the pockets of the separatists on the settlements they occupied through the Sol system. The war saw the use of bio-augmentation protocols for humans to become more adept at the perils of space, which served as a precursor to gene therapy and the subsequent SPARTAN supersoldiers centuries later. The war saw the eventual defeat of both Koslovics and SDF forces, with their remnants that were scattered across the colonies being completely crushed by the new might of the UNSC. 2170 saw the official end of the war with the signing of the Callistro Treaty between leaders of the UN, SDF, and Koslovics.

Aftermath
After the end of the war, the Unified Earth Government (UEG) was immediately established as Earth's governing body by the United Nations. After the cessation of fire, the UNSC found itself dealing with minor yet serious problems: the overpopulation at Earth and it's massive military which had no real enemy to fight against. The post-war period saw surges of mass population that, alongside the destruction and the famine bred brought by the Rainforest Wars.

Humans remained confined on their home system for the next decade, where they would eventually discover slipspace travel in 2191, subsequently forming the Inner Colonies in 2362. Through the means of interstellar colonization, Humanity would enter a new golden age, known as Domus Diaspora, not seen since the beginning of the 20th century, which would also effectively ended the overpopulation on Earth across off-system planets through the Orion Arm.