United Nations Space Command

The United Nations Space Command, more commonly known as the UNSC, is the military, exploratory, and scientific agency of the Unified Earth Government. The UNSC formed halfway through the 22nd century as remnants of old cultural ideologies clashed for supremacy in the Sol system and mainly oversaw United Nations military operations in space. The UN massively militarized its off-world colonies via propaganda and then defeated communist and fascist forces in the "Interplanetary War" which consisted of several side-battles that took place on Mars, the Jovian Moons, and the South American rainforests. Although the Interplanetary War brought great suffering and death unto Earth and its colonies, it united humanity's militaries into a common, armed force by the end of the 22nd century

Humanity was in chaos before the First Battle of Harvest in 2525. The UN was waging a bloody struggle against groups of terrorists (or freedom fighters) called the Insurrectionists, who wanted independence from the Unified Earth Government. The UNSCDF, the military branch of the UNSC, constantly battled the Insurrectionists. In an attempt to help end the long-running war against the rebels, the UEG commissioned the ORION Project (also known as the SPARTAN-I project) and later the SPARTAN-II Program, which created a group of super-soldiers to combat the separatists and the insurrections that they spawned. When the Human-Covenant war began, and the technologically superior aliens began decimating the Outer Colonies, these SPARTANs became humanity's best hope for survival. Faced with genocide on an unprecedented scale, the UNSC mobilized for total war.

The dire circumstances of the Covenant conflict allowed the UNSC to override civilian rule and establish itself as humanity's primary government. Although the Unified Earth Government was more open to step down, the CAA, the arm of the UEG ruling over the colonies, resisted the UNSC's rise to power, and was thus stripped of its power.

By the time the war began, humanity had reached Tier 3 of the Forerunner civilization ranking system. Technological improvements continued to rise due to the capture of advanced Covenant equipment, which was studied and adapted for human needs. The very best and cutting edge of UNSC war assets were supplied to the SPARTAN-II Program. Other enhancements geared towards mass production were distributed to the SPARTAN-IIIs and IVs. One such example of war time innovation was the energy shielding adapted and improved for use on the MJOLNIR Powered Assault Armor. UNSC Infinity, built near the end of the conflict, was perhaps the height of UNSC technical achievement during this troubled period. Incorporating every scrap of technical knowledge that had been scavenged and reverse-engineered from Forerunner ruins, the Infinity stood apart as the most formidable warship and exploration vessel in human history.

By 2552, after the fall of the military fortress world Reach, Earth was the last remaining human bastion, and nearly all surviving UNSC forces were committed to its defense. The Battle of Earth saw some of the greatest bloodshed of the entire war, both in space and on the planet's surface. For several weeks, only a grueling campaign of attrition with heavy participation by surviving Spartan units stood between mankind and extinction. After a desperate struggle, the Human-Covenant war finally ended after a combined UNSC and Covenant Separatist strike force eliminated the Covenant leadership and destroyed the Flood threat.