noun
main fleet
Refers to the principal naval force of a country or operation, as opposed to auxiliary or detached squadrons.
連合艦隊の主力艦隊が真珠湾を攻撃した。
The main fleet of the Combined Fleet attacked Pearl Harbor.
艦隊 is a general term for a fleet or naval squadron, while 主力艦隊 specifically denotes the main or core fleet.
Compound of 主力 (main force) + 艦隊 (fleet).