The Marines as depicted in the game are the force mounted by the TSF agency. The Trans-System Federation (TSF) 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 TSF 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 TSF'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 TSF alive. The TSF 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 TSF 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 TSF 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 TSF. 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" TSF 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. 
The Marines are rigorously trained, versatile soldiers that can perform many vital functions, such as using the Build Tool to assemble unbuilt structures, repairing damaged or broken Power Nodes, and becoming Marine Commander at an unoccupied Command Station. Marines can be given Waypoints by the Commander to coordinate squad movement and actions.
The Frontiersmen arsenal contains a wide array of weapons, purchasable using Personal Resources in the Buy Menu of different structures. Provided by Alterra Corporation, Frontiersmen weaponry were initially designed for human warfare. Since encounters with the hostile aliens have persisted over the past decade, the Frontiersmen have added adaptations of their own to combat the Kharaa threat.
All Marines are armed with the standard Assault Rifle, Pistol and Switch-Ax. Additional weapon types require research by the Marine Commander at their respective structures. Some weapons weigh more than others, decreasing Marine movement speed when equipped.