Prologue.
Everyone in the UNSC is aware that the Human-Covenant War isn't going too well for the human side. The people in the outer colonies were the first, followed by those in the farther inner colonies. These people know better than anyone. Having your family and neighbors slaughtered wholesale right in front of you by a merciless enemy and then having said merciless enemy turn your planet into molten glass right under your feet while you are racing for orbit and a UNSC evacuation to a planet closer to Earth that you hope won't be the next stop on the Covenant juggernaut through human territory gives you a unique perspective on just how badly we are doing. A perspective shared in totality only by the men and women of the UNSC dying everywhere around you, trying to give you the chance to escape.
During this conflict that has raged across star systems for the last twenty five years, the United Nations Space Command Defense Forces have taken incalculable losses. The UNSC Marines and Navy have been the hardest hit; they have comprised the majority of combatants on the front lines attempting to stall the Covenant's advance. Facing severely depleted manpower, HIGHCOM began taking steps to more fully integrate the UNSC Army and Air Force into this conflict.
During normal times, Marine ODSTs were the premier rapid reaction forces of the UNSC with contingents assigned to Navy vessels on patrol. The Army and Air Force were normally assigned ground based defense roles, protecting the larger colonies from Covenant ground incursions.
ODSTs are normally backed up by larger Marine elements. Given the ODST/Marine loss rate, their role is now being backfilled by rapid deployable Army units with Air Force aviators taking on roles traditionally filled by Marine and Navy aviation units.
UNSC Army and Air Force Special Forces have already been mobilized, including the entirety of the 75th Ranger Regiment, 82nd and 101st Airborne and Air Assault (respectively) Divisions as back up for the SF units.
About two years ago, with the threat of extinction at the hands of the Covenant growing more real after every engagement, HIGHCOM realized they were going to have to start retasking the remaining Army and Air Force units that were still mission capable, breaking them down into more mobile and rapidly deployable combat units, supplying them with the same tier equipment that the previous units were equipped with, and giving the Air Force their own ability to get these units to the front.
Flush with funding for advanced training, rapid re-equipping and rearmament, UNICOM (Unified Ground Command) evaluated their assets available and after the retasking and deployment of their current Special Operations capable assets, they identified, reorganized, re-equipped, and retasked the next most viable unit to take on rapid deployable, high risk, and high priority missions.
The UNSC turned to the historically elite, they turned to the Cavalry.
This is their story.