In a technical formulation, I need to define two opposite states. In particular, the first represents the state of an agent which
invades, thereby called invasion. The second one corresponds to the agent which is
invaded. What noun would reflect the latter meaning?
States are more like adjectives or attributes, and entities move from state to state is more like some thing, a noun, having an attribute corresponding to the state the entity is in.
'Invasion' is the noun for the act (the verb) invading. So for the process of an 'invasion', the 'invaders', a noun/entity, the actors preforming the invasion) invade (a verb/ the label for some transition/change of state) a ... well, you're looking for some word that says it is the entity that has been invaded, the object of an invasion.
There is no single word, cognate or not, that captures the object of the invasion.
Invaded land, Conquered people, occupied territory, the place that has been invaded
are all noun phrases which capture the idea, but a single noun doesn't exist for it in English. English doesn't really allow well for making a noun out of an adjective.
In Romance languages, you can use adjectives without a following noun:
which has been borrowed directly into English as 'employee'. The '-é' or rather the '-ee' ending is also productively in English but it takes time for particular instances to catch on. Divorcee, trainee, adoptee are all objects of a transitive verb. 'Invadee' ... just isn't an accepted word in English at least not in formal writing.