The tension was gradually building in the first half of season three of The Expanse, an all-out war between Earth and Mars was ever more likely. All it takes is a spark to ignite the flame.
Put that all to a head in episode five “Triple Point” when a United Nations Navy fleet and naval forces from the Martian Congressional Republic rendezvous at Jupiter.
On the UNN Agatha King (T-DRN-32), Admiral Souther is chafing under the leadership of Fleet Admiral Nguyen. After receiving a video detailing the conspiracy implicating the undersecretary of the United Nations Sadavir Errinwright.
It would seem as if Nguyen is doing anything Errinwright tells him in order to further the war with Mars. In response, Admiral Souther relieve Admiral Nguyễn of command, charging Nguyễn with conspiracy to wage an illegal war.
Admiral Souther then orders the Agatha King and the UNN’s Jupiter Fleet to stand down; however, Nguyễn retakes control the ship and kills Souther, ordering the Jupiter Fleet to return to the AO and prepare to engage the approaching MCRN fleet. Some will follow his orders, others won’t.
Admiral Sandrine Kirino on the MCRN Hammurabi is watching things unfold as the UN ships begin firing on each other. She declares that any UNN vessel that stands down will not be targeted and will be recognized as flying under a flag of truce, prompting Nguyễn to remotely launch stealth pods containing Protomolecule hybrids towards Mars from the surface of Io.