[Sentinel]

United Nations Space Command Defense Force
The United Nations Space Command Defense Force is a division of the UNSC tasked with military matters. It operates under the umbrella of the United Nations Government, though where the distinction is made between the three organisations (UN, UNSC and UNSCDF) is unclear. The UNSCDF operates in two distinct, general divisions, Navy and Marines, with several subdivisions existing throughout the two.

United Earth Space Corps
The United Earth Space Corps, more commonly known as the "Marine Corps" is the main ground combat arm of the fictional UNSC organization of the Halo universe. The Marine Corps serves as a versatile combat element, and is adapted to a wide variety of combat operations. The Marine Corps is composed of infantry combat forces serving aboard naval vessels, responsible for security of the ship, its officers and crew, offensive and defensive combat during boarding actions, and carrying out amphibious and planetary assaults. The Marines are the main ground force component of the UNSC, and are extensively trained in the usage of all UNSC and Covenant firearms, as well as the operations of ground vehicles used by both sides.
The standard rifle of a UNSC Marine prior to October 2552 was the MA5B assault rifle. As of Halo 2, the Marines are issued a BR55 rifle. Marines typically carry an M6C magnum or M6D pistol as a sidearm. In close-quarters battle Marine fire-teams will also make use of the M90 shotgun and M7 Submachine gun. Marines from both games are also armed with the venerable M9 HE-DP fragmentation grenade. Marine sniper weaponry includes the S2 AM sniper rifle. Anti-armor duties are handled by the M19 SSM (SPNKr) rocket launcher or the M41 SSR MAV/AW rocket launcher.
Marines have been seen wearing Jungle, Desert, and Urban Battle Fatigues and ballistic plate armour made of an unknown metallic substance, but it will not hold against a direct shot from the plasma weapons of the Covenant. In Halo: CE, this armour visibly resembled bronze, but in Halo 2, the armour is now green and notably more advanced. Halo 2 Marine armor is lighter than its Halo CE counterpart, and features an array of pouches, with fewer heavy plates for improved mobility, as well as a sidearm holster.
In Halo, Marines are seen with a green tactical display over one eye, attached to the helmet. Text can be seen scrolling across it, highlighting the use of the screen as an instant tactical/recording display. In Halo 2, the screen is gone, but it is likely that the text is now on the helmet's visor. In addition, the helmet links up with the Marine's standard issue neural implant to provide real-time tactical and health data to officers and fellow Marines.
Marine armor support consists of Warthog light reconnaissance vehicles and Scorpion tanks. Marines, their equipment, and their vehicles are typically deployed via Pelican transports, which act as light landing craft and assault vehicles, analogous to the modern-day CH-53 Sea Stallion helicopter.
The UNSC Marine Corps is modelled after the United States Marine Corps and shares many similarities and traits with the modern Corps. A common similarity is the usage of the marine battlecry of "Hoo-rah!". Interestingly, the UNSC Marines also use the United States Army battlecry of "Huah!", and individual Marines are alternately called "marine" and "soldier." Combined with the apparent lack of a UNSC Army branch, this indicates that various international army units have been incorporated into the United Earth Space Corps. It is also insinuated, but not confirmed, that the UNSC Marines share a similar unit structure to the modern Marine Corps.
Notable Marines include Sergeant Avery Johnson, Private Chips Dubbo, Sergeant Banks and Corporal Perez.

Orbital Drop Shock Troopers
Main article: ODST
The O.D.S.T. is an elite special forces group of the UNSCDF Marine Corps, commanded by Naval Special Warfare and Marine Special Operations Command. They are dropped into their mission areas by an HEV (Human Entry Vehicle)'s low-orbit drop. They are also known as Helljumpers. It is an all volunteer outfit, and as mentioned in the book Halo: The Flood, it takes a "special kind of crazy" to join. They are analogous to all types of Special Forces personnel such as Marine Force Recon, Army Rangers, Delta Force, and Navy SEALs.
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Known UESC Marine Units:
· 405th Marine Division
o A Company, presently stationed at Diego Garcia.
· 105th Shock Division
o NSWG2-Team 5 "Helljumpers"
§ 1st Platoon, In Amber Clad detachment

UNSC Navy
The Navy is the space combat element of the United Nations Space Command Defense Force. Its roles include ship-to-ship combat, orbital bombardment, the deployment of Longsword atmospheric and space fighters, and the method of delivery for Marine and Special Operations (ODST) forces through use of Human Entry Vehicles (HEVs). The UNSC currently has a significant fleet force, along with Longsword fighters, though much of the UNSC Navy was destroyed during the Fall of Reach. Currently, UNSC ship production is handled by the Reyes-McLees Martian Shipyards.
The primary offensive armament of UNSC spacecraft are Magnetic Accelerator Cannons (MACs). These are high velocity coilguns that are used to accelerate projectiles ranging in mass from 600 (metric)tons in the case of smaller warships, and 3,000 tons in the case of Super MAC platforms. These projectiles are composed of Titanium-A, depleted Uranium, and/or Tungsten with a ferrous (iron) core. In Super MAC platforms, they are accelerated to over 40% of the speed of light by twenty-three linear accelerator coils. Ship-based MACs have a lower velocity. The UNSC Frigate Gettysburg is quoted as firing a 600 ton shell at 30 kilometers per second. Before Reach fell, the Pillar of Autumn was equipped with a modified MAC that could fire three lighter projectiles instead of just one heavier projectile. It was originally intended to allow the Pillar of Autumn a better chance of disabling a Covenant ship, which was a feature required for a mission that was later scrubbed in Halo: The Fall of Reach.
UNSC warships also carry Archer missile pods for ship-to-ship engagements, which prove themselves most effective against unshielded targets. A common tactic among groups of UNSC ships is to hammer the target's shields with MAC weapons and then pummel it with Archer missiles.
UNSC ships also carry "Shiva" nuclear warheads, an advanced form of nuclear missile that is able to overcome the limitations of nuclear missiles in space, capable of generating EMP bursts even in vaccuum.
As well as these, the UNSC mounts 50mm MLA CIWS for defense against enemy fighters and boarding craft, controlled by shipboard AIs.
UNSC warships are also equipped with significant Marine detachments for security, boarding actions, and planetary assault, and carry all the armor and firearms that could be needed by ground forces. It is implied that a significant portion of the interior of UNSC warships is devoted to this purpose.
UNSC ships also carry a number of C-709 Longsword Interceptors. These are armed with 110mm gatling Cannons and Anvil-10 ASMs. Alternatively they can be radio controlled and used to carry a Shiva that will detonate on impact, effectively destroying both the Longsword and its target.
Despite all of this firepower, however, UNSC warships can seldom defeat Covenant ships in an even engagement. It typically takes multiple hits from a MAC weapon to disable a Covenant ship's shields, and Archer missiles must be used in tremendous amounts to breach Covenant shield systems. Even with their shields down, Covenant ships have been noted to withstand direct hits from normal MAC rounds, and even their light ships can survive direct hits from Shiva nuclear missiles. This advantage in firepower and shielding gives the Covenant an effective 1:3 ratio or better against UNSC ships.