[dman]

How do you classify overt and covert military activity by a nation whose territory has been annexed towards the nation that did the annexation? Is it an act of aggression?

[bzbarsky]

Given no more information than that, I would say the annexation was the act of aggression and the action in response is defense.

In fact, annexing another country's territory without any provocation is pretty much the definition of "a war of aggression".