The United Nations Space Command is tasked with overseeing the safe expansion and protection of the Galactic Republic's territories; including all systems, planets and space colonies. The UNSC was created during the Human-Covenant War and is run by the United Nations security council. It's professional head is the UNSC High Command, based on Earth. Control is exercised through regional CENTCOM's, which have responsibility over assigned areas of space. The Navy houses the divisions of UNSC Marine Corps, Office of Naval Intelligence (ONI), and the Fleet. The UNSC Army and UNSC Air Force maintain garrisons on colony worlds.
Unlike the Covenant, UNSC military technology is still based on comparatively primitive, but effective, mechanisms - which include projectile weaponry, wheeled vehicles, and combustion engines. The UNSC's grasp of scientific concepts is far more advanced than the Covenant, but turning these concepts into functional machinery has proven more problematic. Not so for the Covenant.