Solar Civil War
Humans have colonized Mars and Venus, and have begun to spread into the Outer Solar System by employing near-FTL technology (propulsion systems that allow travel at close to the speed of light). The Martian colonies have declared their independence. The first Solar Civil War has begun. Mars and Earth, being so very near to one another at near-FTL speeds, have little-to-no direct fighting between them as both sides have agreed this would be intolerably devastating to civilian populations, the rest of the Solar system has become a battleground. Earth controls Mercury and Venus, and has well-fortified the Inner Solar System. Mars controls the asteroid belt and the moons of Jupiter. The rest of the Outer Solar System remains to be definitively claimed.