system
Given the following post, respond as if you're commenting on Reddit:
human
[The Expanse] how does geopolitics work when everything keeps moving around?

(Disclaimer: I have seen the tv show but have not read the books. I plan to read them as soon as work and kids stop interfering.)

So Earth and Mars is locked in a cold war, where they both try to maintain influence in The Belt and deny the other acces to the ressources there. How much force each power can project with their navies matter a great deal, but everything moves around so there’s no way to maintain a sphere of influence. At one point Ceres station is 2.5 AU’s from Earth, and 3 from Mars. A year later Mars is 0.5 AU from Ceres, and Earth is 3 AU from Ceres. Suddenly Mars is in a great position to blocade Ceres, and a year later again the position is reversed. 

Is there also a threat of mutually assured destruction when missiles (even equiped with the Epstein drive) have to travel for several weeks to reach the heartland of your opponent? Wouldn’t there be plenty of time to shoot down the missiles and deploy a fleet? If Earth still have the largest navy, and Mars have no way to  guarantee retribution, why haven’t Mars already been destroyed?
gpt
So yea, your 'sphere of influence' isn't going to be a literal geographical sphere as it sometimes is with land-based strategy.  How fast you can project power to where is going to be in constant flux, but that flux is highly predictable.  It can be planned for and accomodated in your overall strategy.  
If Earth knows their supply line is about to get longer, they adjust by stockpiling and beefing up defenses around transfer orbits from Mars.