army (n.)youdaoicibaDictYouDict[army 词源字典]
late 14c., "armed expedition," from Old French armée (14c.) "armed troop, armed expedition," from Medieval Latin armata "armed force," from Latin armata, fem. of armatus "armed, equipped, in arms," past participle of armare "to arm," literally "act of arming," related to arma "tools, arms" (see arm (n.2)). Originally used of expeditions on sea or land; the specific meaning "land force" first recorded 1786. Transferred meaning "host, multitude" is c. 1500.

The Old English words were here (still preserved in derivatives like harrier), from PIE *kor- "people, crowd;" and fierd, with an original sense of "expedition," from faran "travel." In spite of etymology, in the Anglo-Saxon Chronicle, here generally meant "invading Vikings" and fierd was used for the local militias raised to fight them.[army etymology, army origin, 英语词源]
invade (v.)youdaoicibaDictYouDict
late 15c., from Middle French invader "to invade," and directly from Latin invadere "to go into, enter upon; assail, assault, attack" (see invasion). Related: invaded; invading.