The Frontiersmen, often referred to as Marines, are rigorously trained and highly disciplined soldiers enlisted by the Trans-System Federation for the crucial challenge of confronting the hostile alien species known as the Kharaa. When the presence of Bacterium is detected in facilities and spacecrafts, Marine teams are dispatched in rapid response to secure the locations and confront the inevitable alien onslaught. Armed with advanced weaponry and equipment, the Frontiersmen are humanity's first line of defense against the Kharaa.
The Marines are versatile soldiers that serve many functions, such as using the Build Tool to assemble unbuilt structures, become Commander at an unoccupied Command Station; purchase weapons from any Armory or Prototype Lab via the Buy Menu, repair damaged or destroyed Power Nodes, and lock doors temporarily or weld them shut. Marines can be given waypoints by the Commander to coordinate squad movement and actions.
The Command Station can be entered by a Marine who takes the role of a Commander to gain a strategic overview of the map, and the ability to coordinate individual Marines or squads via Waypoints. The Commander is able to place unbuilt structures anywhere, which either Marines or Mobile Automated Constructors must assemble, as well as an active Power Node in the immediate vicinity to provide power to the structures.
In addition, the Commander is also tasked with advancing Frontiersmen technological research such as weapons, equipment, and attack and armor upgrades; unique to his role, the Commander may provide immediate assistance to Marines both in and out of combat, using Personal Resources to create Medpacks, Ammopacks, and even fully functional weapons anywhere. Along with special structure energy-based abilities such as Nano Shield, Scan, and Nano Construct the Marine Commander is able to turn the tide of a battle with good decision-making.
Team Resource is used by the Marine Commander for building structures, producing mechanical units, researching upgrades, and using commander abilities.
Most Marine structures require power to operate, therefore the Marine Commander must reroute power from a Power Socket by installing a Power Node. Power Sockets are located in most rooms and corridors on a map, and either Marine or Mobile Automated Constructor must manually assemble the unbuilt Power Node to activate the Power Grid.
The Power Node cannot be removed once installed; however, take caution as the Kharaa can severely damage a Power Node, which causes power outage and unpowering most Marine structures within the area. An inactive Power Node can be fully repaired by Marine or MAC to reboot the Power Grid.
The Marines as depicted in the game are the force mounted by the TSA agency. The Trans-System Authority (TSA) is an independent body funded by 37 trans-gov powers (governments or super-corporations that own and operate Phase Gates between solar systems), and given jurisdiction over trans-system affairs: as defined by a document called The Charter. The Charter is a comprehensive document, covering disarmament, fair trade agreements (most importantly, sharing phase gates), and certain basic civil rights that had become blurred (or tossed aside) during the dark times of the Expansion.
To enforce The Charter, the Trans-Govs chose Admiral Rathine Studaber, former head of the revived British Secret Service, and gave her a budget for a small military force – which proved hard pressed to stop those same greedy and quarrelsome powers from violating almost every agreement they had made: often concerning their own citizens (termed "population abuse"). It was not very long before the TSA had to resort to military police actions. Its surprising success against the vastly more powerful and far better equipped Trans-Govs earned it the secret hatred of the same powers that created it. They have long been angling to curtail the TSA's powers, slash its funding, or even dissolve it entirely and replace it with something they can control. So far, their distrust of each other, and intense media scrutiny, has kept the TSA alive. The TSA sees itself as the only sane player in trans-system politics, and has become an unappreciated (and often vilified) champion of human rights. 
With the failure of corporate and government forces to contain the spread of the Kharaa, the opinion (among those in power) was humanity should just quarantine the entire Ariadne Arm (the string of solar systems where the Kharaa were encountered). The TSA found this intolerable, both for the people trapped in the Arm, and for the future of human expansion (the economic blow of losing all those ships and bases could threaten the tenuous peace of the Charter). The TSA volunteered to mount a response to the Kharaa – effectively creating a new army, with a new mandate. The resolution passed, and the new force was called the Frontiersmen: "standing on the edge of the unknown, between all of humanity and whatever would threaten it." The Frontiersmen have clearance to travel anywhere, and board any craft. They have rapid-response outposts scattered throughout the Ariadne Arm, with teams of marines ready to deploy. The Frontiersmen initiative is a huge opportunity for the TSA. It's also a risk: if the Frontiersmen fail, on such a public and historic stage, the Ariadne Arm will be signed off as a loss and the trans-govs may have the excuse they've been looking for to "re-organize" TSA leadership – replacing the Admiral and her staff with corporate lackeys. If that happens, dark days are sure to follow. 
Frontiersmen must pass rigorous physical and psychological exams before they can even enter training. The training process, though in many respects similar to militaries all over the galaxies and all throughout history, is unique in its emphasis on small squads, tactical flexibility, and the unprecedented nature of the enemy Frontiersmen train to confront. Frontiersmen are trained to enter a new environment in squads of 5 to 15 individuals, with very little equipment or support, and achieve definitive victory against a virulent and hostile xenoform foe. 
|Mobile Automated Constructor|
- In NS1 but without information on inclusion/exclusion in NS2:
- Light Armor
- Turret Factory
- Not brought over from NS1:
- Light Machine Gun - Replaced in role of default primary weapon by Assault Rifle
- Heavy Machine Gun
- Grenade Launcher - Although similar functionality with Grenade Launcher Attachment
- Grenade - Although similar functionality with Grenade Launcher Attachment
- Phase Gate - Not initially planned for NS2, but was implemented in patch 178
- Knife - Replaced in role of melee weapon by Switch-Axe