The Expanse is set hundreds of years in the future where the Solar System has been colonized by humanity. The United Nations, the Martian Congressional Republic on Mars, and the Outer Planets Alliance (OPA), a loose confederation of the asteroid belt and the moons of Jupiter and Saturn, are embroiled in a cold war. The belters are an oppressed and mistrusted working class that provide many goods for Earth and Mars through the mining and other work facilities built on the asteroids. Although they are a weaker, less organized faction, they have started to resist the powers of Earth and Mars. Eventually an interplanetary war breaks out between the UN and the MCR Against the belter "terrorists".
Spaceships are equipped with fusion drives Called Epstein Drives and are able to easily make interplanetary trips.
Terraforming of Mars is halted once a Ring gate is discovered that gives humanity access to other solar systems with habitable worlds. The Ring gates are artificial wormholes that enable faster-than-light travel between star systems. These were created by the Ring Builders.