Basically , Eathforce (the EA military) is a single , combined arms , force . It is composed of several branches:
- The fleet (basically a mix of navy and air/aerospace force)
Earth fleet is basically an aerospace/deep space force , heavily based on old , naval traditions . It has a small naval infantry ("marines") force , basically for boarding actions and onboard security .
- The Planetary forces : (army+marines+naval/sea forces+atmosferic/air force) .
The planetary forces are responsible for all sorts of... planetary operations . From typical ground actions to maintenance of air or sea superiority , up to the orbital level (orbital assaults -Marine style included) .
Hope this helps you
