[dman]

By this definition is Ukraine the aggressor in eastern Ukraine?

[bzbarsky]

By which definition? The "you could argue" definition that I don't agree with? Or my "personally, I would consider" definition?

By the latter, if country A is arming/training/providing insurgents in country B with no previous attacks by B on A, then A is the aggressor. So in the eastern Ukraine case Russia would be the aggressor.

[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".