On December, 2163, the UNC or UNSC was initiated, with the Argyre Planitia Campaign. The United Nations Command and the Frieden and Koslovic commanders signed the Callisto Treaty in 2170, ending the heavy fighting. The United Earth Government or UEG was established that same year as a direct result of the Interplanertary War.
Before the beginnings of the war with theCovenant, humanity was in turmoil, with the remote colonies fighting for independence from the UNSC. To help quell the revolts, the UNSC commissioned the Spartan Project, which created elite super-soldiers called Spartans. The job of these soldiers was to stealthily eliminate rebels and insurrectionists. When the Covenant began decimating the outer colonies, these Spartans became humanity's best hope against the technological superiority of the Covenant.
One of the UNSC's Special Forces units are the Orbital Drop Shock Troopers. They are also called ODSTs or Helljumpers. They specialize in orbital combat insertions via pods (carrying only a soldier and his equipment) dropped from ships in low orbit. The 105th Marine Expeditionary Unit (SOC) are labeled as the 'Helljumpers'