UNSC Marine Corps/Gameplay

The UNSC Marine Corps and its personnel serve as the primary allies throughout all of the Halo games. However in Halo: Reach they are rarely seen due to the main NPC ally in-game being the UNSC Army.

Halo: Combat Evolved
In Halo: Combat Evolved, most Marines carry MA5B Assault Rifles, though occasionally they can be seen with a Sniper Rifle (as seen in Assault on the Control Room and Halo) or a Shotgun (as seen in 343 Guilty Spark), and very rarely a Needler or Plasma Rifle (as seen in Assault on the Control Room and Truth and Reconciliation) when no UNSC armaments are available.

They are relatively accurate with these weapons, with a hit percentage upwards of 50%, often firing in short bursts with the Assault Rifle, killing a Grunt in one or two bursts, knocking a Jackal's shield off position with repeated bursts, or cooperatively working with other Marines to take down an Elite. Marines also often utilize Fragmentation Grenades (mainly when Ghosts get near or on a group of elites) in combat. They don't seem to be able to use rocket launchers even though there are often rocket launchers around the bodies of their casualties.

Marines will strafe and avoid fire, but will not usually seek cover. They will roll out of the way of a vehicle but they will not pick up different weapons, Health Packs, or additional ammunition. Marines are also good pathfinders, often able to maneuver around obstacles such as boulders, debris, or anything else in their path.

Marines are conscious of fratricide. After you have killed two humans of any type, the rest of the Marines in the campaign level will attack the player. If one kills Captain Keyes, or any other members of the bridge crew, in the campaign level The Pillar of Autumn, Cortana will lock the doors and summon a squad of invincible Marines to kill you.

Marines hitch rides on player-driven Warthogs and Scorpion Tanks but are only capable of piloting Ghosts by themselves, but they are not very skilled doing so, and will often run you over by accident if you are not careful. They will remain on the Warthog if the player leaves the driver seat (although will exit both Scorpion and Warthog on the level Assault on the Control Room if left alone). This is most likely due to the fact that the marines in the Assault on the Control Room level are scripted to exit their vehicle once the player exits it. They are only seen driving by themselves in the cut scene of the Pillar of Autumn and cannot do so during gameplay.

Interestingly, the enemy the Marines are most vulnerable to in Halo: Combat Evolved are the Infection Forms of the Flood. Once one is attached to a Marine, it cannot be shaken off and will kill the soldier. Marines are poor fighters against the Flood and will often take heavy if not total casualties fighting them.

They also go "berserk" when they have taken too much damage. When down to one last bit of health, they will scream something (such as: "I'll take you all on!" or "Who wants a piece of me?!") then rush toward the nearest enemy and fire in extended bursts (5-7 round burst as compared to 2-3 round bursts). This is usually the last thing they ever do, as they are badly wounded by this time and the lack of any melee attack (or the use of full auto with an assault rifle) makes them very vulnerable in the close-range combat, and often end up getting killed after going "berserk". If a "berserk" Marine survives a fight, he will calm down and simply return to normal, but will still "berserk" when attacked again.

Halo 2
In Halo 2, the Marines are much more intelligent and "situationally aware" than in Halo: Combat Evolved.

They are more adept with vehicles, and are able to drive the Warthog, the Ghost, the Wraith, and the Scorpion, albeit a bit clumsily. They can now melee sleeping Grunts and will even attempt assassination kills if they catch the enemy unaware, although they lack the brute strength to kill Elites in this manner. Also, they are now able to take cover behind walls, corners, and obstacles with their backs up against them when under fire, something the players themselves are unable to do.

However, their firearm prowess has decreased, and most Marines are unable to achieve greater than a 25%-50% hit percentage with semiautomatic weapons such as the Covenant Carbine, and are instead more effective with close-range weaponry such as the M7 SMG or with heavy weaponry such as the Rocket Launcher or the Fuel Rod Cannon.

Unlike Spartans and Elites they don't have the new ability to dual wield. However dual magnums are sometimes found next to certain marines or additional weapons e.g. an ODST on Delta Halo with dual pistols.

Although their friendly fire incidents have decreased, contrary to popular belief, Marines do use fragmentation grenades in combat (more likely when betrayed), albeit quite rarely, and are unaware of splash damage from heavy weapons, sometimes firing a rocket into a nearby wall and killing themselves.

The Marines are far more interactive, sometimes speaking to each other while in combat, and wield a greater variety of weapons. The player can also switch weapons with the Marines to make sure that they are well armed or that the player has a good team with him. As in Halo: Combat Evolved, the Marines are aware of backstabbing, and will attempt to kill you after you kill a few of their comrades. The number of Marines that can be killed before the others turn against you oddly seems to vary depending on the weapon used and the way they are shot.

Generally, taking out two marines separately in succession will instantly make them betray the player, unless they are taken out at the same time using an explosive weapon. However, if the player progresses through the level and meets another group of marines a distance away from where the betrayal was performed, they will sometimes be marked as allies once again. Also, if the player does not harm any marines who are shooting at him/her for a certain amount of time the Marine(s) will reluctantly return to an allied condition, or sometimes verbally forgive the player.

Changes from Halo: CE to Halo 2

 * Players can now swap weapons with their marine allies.
 * Marines now have the ability to melee.
 * Marines have the ability to drive vehicles during actual game-play.
 * Marine armor is more green color based rather than grey.
 * Marines' helmets can fall off if shot with a direct hit.
 * Marines no longer have the green screen over their eye.
 * Marines have significantly more dialogue lines.
 * Marines respond to and react to one another's speech in non-scripted ways; if one marine cracks a joke another one will laugh.
 * Marines seem to have about 75% more health.

Halo 3
In Halo 3 they can, once again, utilize the Grunts' weakness for naps by using stealth and hitting them, usually with a kick, but they can also hit something with their rifle. However due to the brute strength of the many races of the Covenant, UNSC Marines will usually lose in melee combat against vicious ballistic creatures such as Brutes or Hunters. They seek cover much more often and even put their back up to the wall. They will throw grenades much more often, and with great accuracy, especially in higher difficulty levels. They will sometimes also display group tactics never seen in previous games, like huddling into a tight group and combining assault rifle fire while combating Drones. Even though the Marines are smarter in Halo 3 they occasionally will make mistakes, such as running off a cliff while driving a vehicle. Marines/ODSTs seem to be able to react with the environment around them. For example, if a Ghost is about to explode, they will stand back or walk around it and continue after it explodes. Despite this, their AI was considered poor compared to the enemy AI (such as the Covenant and Flood). There are also a lot more face and skin models for the Marines in Halo 3.

The player can still exchange weapons and ammo with them, though they won't accept Brute Shots, Gravity Hammers, Sentinel beams or Energy Swords (Except Johnson on the level Halo). Marines in Halo 3 have infinite ammo for their weapons, which can be exploited by giving Marines heavy weapons.

After killing two of them, any that are left in the area will come to kill you. After a while (even if you are constantly killing them) they will see you as a friend again. Interestingly, their accuracy becomes 100% if betrayed, and if there are two or more remaining Marines they can end the Chief's life in a matter of seconds.

Their audio clips have also been vastly expanded, and profanity is used to indicate wounded status if the IWHBYD skull is activated.

Marines are vulnerable to the Flood Infection Forms as they were in Halo: CE. If an Infection Form attaches itself on a Marine, they will grapple for a second or so, to give the player a chance to shoot it off, but if the player does not, the Infection Form will mutate the Marine into a Combat Form. However, it is possible to "short circuit" the mutation by firing on the new Flood host, and cause the human to not fully mutate into a combat form. It is also possible to shoot off the Infection Form, but the Marine will likely take some fire in the process.

Marines are extremely ineffective against the Flood if not given Shotguns or Assault Rifles, and will take extremely heavy casualties fighting them.

Marines are seen in larger groups in Halo 3. In previous games they were usually seen in groups of 4-6, or one squad. Now several squads can be seen at a time, and all are able to assist you.

Marines have also gained the ability to flip vehicles, which compensates well for their somewhat poor driving. They can flip the Mongoose or Ghost by themselves, and can flip the Warthog when in groups of at least three, and strangely, they can flip the 66-ton Scorpion in groups of only five.

Halo 3: ODST
For the most part, Marine allies in Halo 3: ODST are near-identical to their counterparts in Halo 3, though there have been a number of improvements made by Bungie. For one thing, driving and movement AI is far superior, including their pathfinding ability. Accuracy has also been increased, whether on foot with handheld weapons, or using turrets. "Gunners" using the machinegun turret of a M808B Scorpion tank are also improved, with better targeting than their past incarnations.

Halo Wars
In Halo Wars, Marines are built in "squads" of four (or five when you upgraded them with New Blood).

Their starting special ability is a grenade attack which can be upgraded to RPGs. They have up to three upgrades : In cutscenes, marines are very weak and can only take 1 projectile from any weapon to kill or badly injure them depending on where it hit or what was used, however this is likely more in line with the books in terms of realism.
 * 1) "New Blood": Adds one extra marine to each group, increasing the damage they deal over the same length of time and slightly increasing maximum health.
 * 2) "RPG": Upgrades the Grenade Toss active ability to the RPG active ability, which has a longer range and deals more damage.
 * 3) "Medic": Adds a fighting medic to each group, which allows injured Marine units to heal while not taking damage.

In addition, if the player selected Captain Cutter as their Hero, the Marines can be upgraded to ODSTs, which have more maximum health, deal more damage, and have higher defence. In addition, ODSTs can be deployed anywhere on the battlefield with the "ODST Drop" Hero Ability.

Marines armed with the NA4 Flamethrower are built as separate units called "Flamethrowers". Flamethrowers received upgrades such as Flashbang grenade, which stuns enemy infantry temporarily, and Napalm Adherent, which causes enemies attacked by Flamethrowers to burn for several seconds after the initial attack, causing damage over time. Like Marines, Flamethrowers can be created at the UNSC Barracks or trained via a deployed UNSC Elephant, though like the ODST upgrade, the latter is only possible if Captain Cutter is the selected Hero.

Halo: Reach
In Reach, Marines are encountered near the end of the game during the level The Pillar of Autumn. Little is known about the Marines on Reach as the main NPC ally is almost always the UNSC Army. They can be recruited as the player's fireteam to follow the player throughout the level. It should also be noted that Marines, Army troopers , and ODST's melee's are useless against all enemies  in game.

Halo 4
In Halo 4, Marine gameplay has degraded rather significantly compared to how they performed in Halo 3 and Halo 3: ODST, especially in combat. The first notable change is that Marines would often shoot at deceased enemies for a noticeable amount of time, even in the middle of combat, although this is generally only a problem when fighting Covenant, as Prometheans enemies either disintegrate or fall to pieces upon death without leaving a one-piece corpse that can register as a target. It is worth noting that all AI-controlled units do this, enemies included, with the only exceptions being disintegrated targets that do not leave corpses.

Another notable flaw in the Marines' performance is vehicular combat. While Marines were poor drivers in previous games, they are even worse in Halo 4, driving as slowly as possible and not taking any evasive maneuvers in combat, which could spell doom to the player on higher difficulties. And while they are still quite competent when manning a turret, Halo 4 Marines still perform poorly as compared to their past incarnations; aside from firing at deceased enemies mid-combat for prolonged periods, Marines often get distracted by Phantom dropships flying away overhead and ignore the more immediate threats such as a Ghost or a Wraith a distance off on the ground.

Finally, when given heavy weapons, they are generally very accurate and effective, but are still able to blow themselves up in close-quarters scenarios, or betray the player(s) and any other friendlies.

That said, Marines still retain their resilience, able to distract enemy forces for a surprisingly long time before dying, and have scripted invulnerability in a few scenarios, at least until they rendezvous with the player. NPC SPARTAN-IV characters appearing in a few scenarios are functionally the same as Marines, sharing their AI and having similar combat capabilities.

Another good thing about the Marine A.I. in Halo 4 is that if they man a machine gun turret, they can easily kill Promethean Crawlers and even Promethean Knights on Heroic.

It should also be noted that Marine's can kill an unshielded Elite with a melee on lower difficulties such as easy or normal.

Trivia

 * In the in-game game mechanics, Marines actually possess regenerating armor that functions similar to the regenerating energy shields possessed by the Master Chief and enemy Elites. However, unlike energy shielding, this armor does not have special damage values (i.e. it does not take reduced damage from bullets or increased damage from plasma). In Halo: Combat Evolved, a Marine's armor is about 50% as strong as a Minor Elite's shields. In Halo 2, the armor appears to be on par with a Minor Elite's shielding, allowing a Marine to take roughly the same amount of damage as a Minor Elite.
 * In Halo: Combat Evolved, shooting a dead Marine's armor with a ballistic firearm would eventually send a ricochet at your shields. Meleeing them would result in a loud "clack" However, in Halo 2 and 3, the bullets do not ricochet, but penetrate the dead bodies and result in a large amount of blood.
 * In Halo 2, Marines receive a significant upgrade in both A.I. and durability. A Halo 2 Marine can survive as much damage as a fully shielded blue Minor Elite, and use intelligent combat tactics (such as using cover and firing from behind cover) that allow small groups of 1 to 3 Marines to hold their own against large squads of Covenant enemies.
 * In Halo 3, Marines are likewise highly resilient, with a single Marine having roughly the same level of durability as a Brute Minor (on Heroic difficulty a Marine takes about 24 assault rifle shots to kill, whereas a Brute Minor takes 7 shots to drop their shield and an additional 17 shots to kill).
 * In Halo 3, Marines generally tend to care greatly about the welfare of John-117, and become scared and upset if you die. However, when playing Co-op with the Arbiter, if the Arbiter dies, the Marines don't seem to care at all and act as if it's unfortunate, but still unimportant, possibly resentment of Covenant despite them being allies now.
 * In Halo 2, sometimes if you kill ally Marines until they betray you, they will yell phrases like, "I have been waiting for this for a long time!", "You won't get away with that!", or "Kill the traitor!". In Halo: Combat Evolved, they will yell out things like "Let's end this!", or "I've had enough of this crap!"
 * In Halo 3, when Marines approach the bodies of dead Marines, they will crouch beside them, place their hand on the body, and usually utter something like, "I'll make sure your letter gets to your parents," or "Better you than me."If there are groups of Marines dead, other Marines will, upon discovering the scene, utter things like, "What happened here?" or "Poor buggers, never stood a chance." During Crow's Nest, in the Barracks portion of the level, this is a common sight. Marines can also be quite callous when around dead Marines, particularly with the IWHBYD Skull activated, sometimes making statements like, "He had a watch on him," or "I never liked him anyway," or "Where is that watch?" or "I'll miss you, but you said I could have your stereo."
 * In any of the Halo trilogy games, Marines will kill an enemy, and then, seemingly in celebration, shoot the dead body for a few seconds, while yelling things like, "I saw it move!" or "Get up so I can kill you again!"or"How does it feel to be dead!".
 * In Halo: Combat Evolved,Halo 2 and Halo 3 , Marines will taunt the enemy such as yelling to the Grunts "Take off that mask! You can't be that ugly!"
 * In Halo Wars, Marines are a unit. Oddly, although your max population is 40, you can train up to 41 Marines. The same is true for grunt squads.
 * In Halo: Combat Evolved, Marines will sometimes pilot Ghosts. While they can be useful in combat, Marines are terrible at steering the Ghost and they will often run over the player.
 * In Halo: Combat Evolved Anniversary, the Marines will appear as the UNSC Army Trooper in Halo: Reach.

Internal Articles

 * United Nations Space Command
 * UNSC Marine Corps