The Prosperan Republic Interstellar Marines (PRIN) is a subsection of the Prosperan Navy responsible for providing power projection from carriers and space, using the flexibility of the Prosperan Navy to rapidly deliver combined-arms combat specialists.
The Marines have been a component of the Prosperan Navy since the first colonies were founded. The Army was unsuited to space warfare, and only a small section of the Army was trained in the specialized tactics required to effectively commit to boarding parties, boarding defenses, planetary invasions, and zero-g combat. The Marines were founded by the division of this brigade, and evolved into becoming the primary combat force of the Navy, eventually diminishing the role of the Army into primarily garrisons and self-defense forces.
The Marines have around 300,000 members active on carriers, 1.2 million members on cruisers and space stations, and just under 400,000 reservist Marines.
The Watchers are a subset of the Marines, the elite of the elite. They are a special forces brigade that excels in black ops and in 'go silent' operations. In particularly dangerous operations, The Watchers are called in to settle the situation effectively and silently.
Marine tactics mostly revolve around their inherent technological superiority over any potential foe aside from themselves. Even the regular Navy would be at a tech disadvantage in a general war, with so many of the most destructive and efficient weapons being held by Marine armories, including a large portion of Strike Suits, the only thing they would be at a negative on.
With this, the Marines actually are lacking in war experience, with there being no true outside threat. All combat is done against far inferior opponents with the rare rebellion being handled by Watchers- a much smaller group that wouldn't be able to pass on experience as instructors except to other members of the group, which would remain small. Therefore, the Marines' major weakness is thus; their inexperience against an evenly matched foe. However, this has rarely if ever come up and when it does, there is a Prospector to take care of it.
Their technological superiority lets them delete almost every form of protection while staying incredibly protected themselves. Only some Prospector-made weaponry is known to be thoroughly effective against all Marines worn gear.