IPA: /ɪnˈveɪdə/
KK: /ɪnˈveɪdər/
A person or group that enters a place with the intention of taking control or causing harm.
The invader took over the territory without warning.
Invader is formed from "in-" (meaning into) and "vadere" (meaning to go). The word describes someone who goes into a place, often with hostile intent, thus referring to an intruder or aggressor.
Think of someone 'going into' ('in-') a place uninvited or forcefully — that's why an invader is someone who intrudes.