The Martian Congressional Republic is the governing body which rules over the nine billion inhabitants of Mars and one of the two human superpowers. Their flag is reminiscent of Mars and its two moons, Phobos and Deimos.
The Republic is often in direct competition with Earth and its governing body, the United Nations. Although Mars has a significantly smaller population than Earth, the ruggedness of the people, as a result of the harsher environment and sparser resources, makes Mars a worthy counterweight to Earth's power. The Martian Congressional Republic Navy, although smaller, is considered to be more advanced.
When the Donnager is destroyed by what is believed to be OPA ships, Mars declares war on the Belt, and deploys the majority of its fleet to it. Later when Holden released information on the ships that destroyed the Donnager were actually made on Earth, United Nations Navy ships fired on Martian Congressional Republic Navy ships in Mars' orbit and destroyed the Deimos deep radar station. The MCRN then orders all their ships in the system to return to Mars and declares war on Earth.
After the protomolecule was unleashed on Eros during the Eros incident, and its existence became known to the public, the Martian fleet nuked Phoebe into a thin gas that was sucked into Saturn’s gravity.
After the Eros incident, delegates from the UN, MCR, and OPA attend a peace conference of Ceres.
In the eighteen months since the Eros incident the MCR and UN have been in the UN-MCR Cold War.