Marines are the first line of defense for most Terran worlds in the Koprulu sector. In the time of the old Confederacy the vast majority of marines were criminals or rebels who had undergone mandatory neural re socialization. Freed from any previous allegiances or ideologies, these fearless soldiers stood ready to defend Confederate interests with their lives. The intake of "resoc" marines has been scaled back in many places since the fall of the Confederacy, but remains a common practice, for there are seldom enough volunteers for the job.

The heavy armor worn by marines is effective against small-arms fire and provides them with full life support and NBC (Nuclear/Biological/Chemical) shielding for operation in deep space and other hostile environments. Ballistic alloy combat shields have also been developed in light of experiences during the Brood War, giving an option to toughen up marines faced with an increasingly lethal battlefield.

Marines are normally armed with C-14 Impaler gauss rifles that fire 8mm metal "spikes" at hypersonic speeds. Basic rounds are designed to provide maximum penetration against all armor types, but a number of specialized rounds exist. Of these, the U-238 depleted uranium spike is the most popular because it extends the lethal range of gauss rifles by up to 25%. To preserve ammunition usage and minimize power requirements, the rifle uses a capacitor system to fire in short, controlled bursts. This flexible sidearm can be used to target both air and ground enemies.

If the practice of using resocialized marines has remained controversial, the use of stimpacks is even more of a hot-button subject. These inbuilt chemical delivery systems dose marines with a powerful mix of synthetic adrenaline, endorphins, and a psychotropic aggression enhancer. Marines on stims benefit from greatly increased speed and reflexes, but are subject to long-term side effects including and not limited to insomnia, weight loss, mania/hypomania, seizures, paranoiac hallucinations, internal hemorrhaging, and cerebral deterioration. Nonetheless, both commanders and the marines themselves stand by the use of stims as essential to their continued survival and effectiveness on the battlefield.