The United Nations Navy (UNN) Jupiter fleet is mainly based out of Ganymede and Europa. The Truman-class dreadnought UNN Agatha King under the command of Admiral Souther, later replaced by Fleet Admiral Augusto Nguyễn, serves as the flagship of the Jupiter Fleet until the ship's destruction.
At the jointly operated UN-MCR station on Ganymede several UN Marines were attacked by a Protomolecule Hybrid. The UN Marines retreated towards nearby Martian Marins whom engaged the Hybrid unsuccessfully. Communications were being jammed and this led to confusion in orbit with both the UNN fleet and MCRN fleets believing their own forces were under attack by the other. Both fleets engaged causing significant damage to the mirrors in orbit.
After failed peace talks the United Nations and Martian Congressional Republic went to war with each other in the UN-MCR War. With heavy casualties on both sides the majority of the conflict was fought by the MCRN Jupiter fleet and UNN Jupiter Fleet. Fighting occurred at Ganymede, the Jovian system, and over Io during the Io Campaign.