So we all know Earth was destroyed by a giant tentacle monster, and presumably humanity's government was lost there too. Before I thought humans had a democratic government of some sort, and maybe it did before the destruction of earth. But when earth was destroyed the military side of the United systems was all that remained. The USCM, the United Systems Colonial Marines may have been the only government owned branch that remained, which then took official control of humanity and has remained in charge. It seems like the human government is now the military, which has some interesting implications.
IRL, the enactment of the chain of command the USA uses would've taken effect. When one member of the govt dies the next takes their place. President, Vice President, and so on. I'd assume they evacuated atleast a few of the govt members. EDIT: See, http://en.wikipedia.org/wiki/United_States_presidential_line_of_succession
The head cannon that I've been going by is that the majority of the humans that escaped the destruction of earth were the many USCM soldiers that were already off of the planet. There would also be a number of former convicts that escaped their imprisonment after the USCM abandoned the penal colonies. This would mean that the majority of human players would likely be an active duty marine, an ex-marine, an ex-prisoner, or one of the lucky few to escape earth during the chaos of the tentacle monster attack.
Yeah but the USCM represents all of humanity, not just americans. Pretty sure that's why they changed it to United Systems Colonial Marines, not United States Colonial Marines like the original USCM. That said, presumably every country in the world has a similar system to the one you describe. Presumably all the Star-Systems under Humanity's government control were united, hence United Systems
I'd assume the United Systems is like the UN, but oh, hey, look, the UN also has the succession system.