Marine

''This is about the marines that appear in Doom 3. For the marine that the player controls in the series, see Doom's protagonist''.

The Marines are a military organization present (or at least mentioned) in most of the Doom games (except the 2016 Reboot although in this aspect they are replaced by the Elite Guard) they have an expanded presence in Doom 3, where they appear as NPCs, and allies, which are stationed on the Mars base.

Classic Doom
According to the TNT: Evilution manual, the Marines are the military organisation of the United States, in the manual for the original Doom the marines are currently engaged in an unknown conflict, where one of it's commanders ordered his soldiers to fire upon civilians, only to be killed by one of his Soldiers.

In the original Doom series, the player plays as "Doomguy" is part of a marine team, stationed on an unknown Mars outpost, that was sent to Phobos after Deimos vanished from the sky, and Phobos stopped communication after sending a distress call, apart from the Doom Marine, their whole squad is wiped out, leaving the Doomguy as the lone survivor.

After arriving on Earth in Doom 2, the Doom Marine meets up with fellow marines, and soliders to battle the spaceport, but much like the original game, the entire group is wiped out, except for the Doom Marine.

Apart from the first two games, Final Doom reveals that the marines are not as reckless, and incompetent, been shown to wipe out hundreds-to thousands of demons, with no little than 200 men, even being used as "mop up squads" after the invasion of Earth by Hell in the second game, however much like the previous games, Hell was able to out-manuvere the Marines.

In Doom 64 the marines do not appear, beyond the Doom Marine, the only one, sent to the abandoned Phobos base.

Doom 3
The Marines are responsible for security and defense at the UAC facilities on Mars. They are purported to have come from the United Nations Space Marine Corps, which is reinforced by the Doom 3 novels (while not directly stated in-game, their names are all generally American).

In the game, Marine HQ is the hub of activity for the Marines and their subsection, Mars-Sec (short for Mars Security), and is also one of the main air and space traffic control centers for Mars City. Marines and Security personnel are stationed and/or patrol throughout the base, in and around areas including the EnPro Plant, Alpha and Delta Labs, the Monorail, and most visibly in Mars City where the Marine HQ and the primary Armory is located. In the character's PDA, a video disc about the Marine's role on Mars states that all Marines are under the command of Master Sergeant Thomas Kelly, who tries unsuccessfully to contain the invasion before being corrupted.

The Marine Corps also apparently possesses a number of spaceships of its own, which the player is commanded by Kelly to call for backup from the UAC Base's primary communications center. Whether or not the player sends the transmission himself, it is still sent - and when the Earth fleet finally arrives to back up Mars, they find the planet wrecked, and only one Marine left alive who had almost single-handedly stopped the entire demon invasion.

Classic Doom

 * Doomguy (The Player)

Doom Novels

 * Flynn Taggart (Doom Novels)

Doom 3/Doom 3 Novels

 * Doomguy (Doom 3)
 * John Kane (Novels Protagonist)
 * General Hayden (General of the Mars Marines, Doom 3 Novels)
 * Sergeant Thomas Kelly (Master Sergeant, Commanding Officer)
 * Bill Tyson (Staff Sergeant)
 * Frank Cinders (Private First Class)
 * E. Webb
 * K. Miller
 * M. Marak
 * S. Holmes
 * D. Morton
 * S.L. Medley
 * K. Lee-Creel
 * E. Grafton
 * T. Washington
 * M. Ryden (bot technician)

Other Doom Sources

 * Stan Blazkowicz (Doom RPG Protagonist)

Classic Doom

 * Player's Squad (Doom Manual)
 * Spaceport Assault Squad (Doom 2 Manual)

Doom 3

 * Bravo Team
 * Fireteam 2
 * Fireteam 4
 * Recon Zulu (Earth Fleet, first unit to enter and scout the UAC base. Found and rescued the sole survivor, a Marine)
 * Security Team 2
 * Monitoring Team 2

Classic Doom

 * Pistol
 * Shotgun
 * Chaingun
 * Plasma Rifle
 * Rifle (Unusable by the Player)

Doom 3

 * Flashlight
 * Pistol
 * Shotgun
 * Machine Gun
 * Grenades
 * Chain Gun
 * Plasma gun
 * Rocket launcher
 * BFG 9000
 * Security armor
 * Automated Turrets
 * Sentry Bot
 * Grabber Gun

Trivia

 * There are Doom 3 mods that enable the player to have Marines on their side to help fight the demonic forces, similar to the Security Guards from Half-Life.
 * The Marine staff is stated to have increased in Doom 3: Resurrection of Evil; this is supported by more dead Marines in the expansion.
 * A reoccuring gag (known as the "Useless Allies" trope) in the Doom series, is that most, if not all unpossessed marines, except for a certain few (such as Doomguy) are shown to be extremely incompetent, and reckless, being killed by weaker demons and possesed security guards extremely easily.
 * The Marines in the Doom games are inspired by the United States Colonial Marines (USCM) from the Alien series.