"The Space Marine Corps (SMC), is the space warfare branch of Earth's combined government that acts as an expeditionary force in the counter-invasions of Stroggos in retaliation for the Strogg attack on Earth. Comprised of exceptionally brave soldiers, the S.M.C. is thrown into the bloodiest battles humanity has known and is ready to take massive casualties in often near-suicidal operations to defend the Earth.

Presumably its origins lie in the Global Defence Force or GDF, the main body of Earth's defending armies that repelled the Strogg invasion. However, the S.M.C. is much better equipped, with planetary assault ships and drop pods used to invade the Strogg homeworld. It is unclear how the SMC was formed but it may have been created as a response to the Strogg invasion, and its assault ships and much of its weapons may have been developed using captured Strogg technology from the invasion."