During peacetime, the EA fleets are split between the Main Fleet, which is a rapid-reaction force centered around Sol, and the Home Fleet, which is tasked with keeping peace in the often-rebellious outer planets.
When the EA invaded the GTVA, the Main Fleet (CO Admiral Raul Falon, with the Nemesis as flagship) spearheaded the offensive into Delta Serpentis, and then into Beta Aquilae. When the GTVA refused to surrender after losing its capital in Delta Serpentis and nearly losing Beta Aquilae, and GTVA counterattacks threatened the EA's precarious position and long supply lines, the Main Fleet was forced to split in two to wage war in both systems. The Main Fleet itself was devoted to cleaning up GTVA resistance in Delta Serpentis and guarding against a hypothetical counterattack from Ross 128. The bulk of the Main Fleet was split off into the Advance Fleet (CO Adriannos Spiros), dedicated to prosecuting the attack in Beta Aquilae and bringing the GTVA to the peace table.
The EA High Command has kept several Home Fleet battlegroups in Sol out of the action, both as a strategic reserve and as a final defense of the Sol-DS node against unexpected GTVA breakthroughs at the Sol Gate or new Shivan incursions.
The EA is fighting a multi-theater war its fleet isn't prepared for. They were hoping to quickly crush the Terran GTVA's will to resist with a blitzkrieg against the Terrans' two most valuable systems, but the Terran GTVA's stubbornness and High Command's paranoia has split their mighty fleet among three systems.