Series Sci-Fi series about early human expansion into the solar system. At the time of the start of the series, humanity had major colonies on Luna, Mars, and several major asteroids. The two main nations are the UN representing Earth, and Mars. The Belters under the OPA (Outer Planet Alliance) are a third faction, but are not a recognized nation. Warfar is done with conventional weapons (projectile bullets, missiles, no shields or lasers etc...). Espion drives allow travel across the solar system but humanity is still limited to within a single Solar System. Space travel is mostly limited to how high gravity the humans inside can actually handle. Military operations often use special drugs to keep the pilots alive during high-G maneuvers.
This is mostly a dystopian future with large corporations ruling the world with most of Earth on basic living income because there are only enough jobs to keep 1% of the population employed, and most of the belt populated by poor angry Belters. Humanity does not have any way to manipulate gravity, so most of the Belters have severely altered bodies to live outside of gravity. Most of them have elongated bodies and cannot handle Earth gravity anymore. Mars has lower gravity but still has some gravity so the effects are not as pronounced on them. The OPA is a loose alliance of belter terrorists trying to earn equality for themselves.