The year is 2543 or thereabouts and man has colonized the solar system.
There is no FTL travel.
Earth is the most populous planet. The moon is independent. Mars is second. Of course the two are fierce rivals. There is also the Jovian League of Jupiter's moons, the Saturnalia Alliance and of course the Belt. Mercury and Venus are colonies of Earth.
If the reader(s) think we're ripping off The Expanse, the reader(s) are right. The difference is in this universe a bellicose alien species shows up, forcing Earth, Mars and company to come together.