Humanity was in chaos before the Covenant attack on Harvest in 2525. The UN was fighting a bloody war against groups of terrorists (or freedom fighters) called Inssurectionists, who wanted independence from the United Earth Government. The UNSCDF, a branch of the UNSC, constantly battled the inssurectionists. In an attempt to end the long-running war against the rebels, the UEG commisioned the ORION project (otherwise known as the SPARTAN-I project) and later the SPARTAN-II Program, which created a group of elite super-soldiers separatists and insurections where 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 prepared 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 Colonial Administration, 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 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 Infini, the y stood apart as the most formidable warship and exploration vessel in human history.
By 2552 A.D, after the fall of the fortress world Reach, Earth was the last remaining Human bastion, and nearly all surviving UNSC forces were commited 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 surving Spartans sttod 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.
The United Nations Space Command was created by the UN during the Inter-Planetary war of the mid-twenty-second century. after the UNSC's victory, overpopulation on Earth became a major issue in the early twenty second century. The issue was resolved by colonizing and constructing vast cities on the planets in the Orion Arm of the galaxy, the Inner colonies and later, the Outer colonies.
The UNSC's defensive militia force is the UNSC Defense Force (UNSCDF), which consists of the UNSC Air Force, the UNSC Army, the UNSC Marine Corps and the UNSC Navy.