Aggression, n. [L. aggressio, fr. aggredi: cf. F. agression.]
The first attack, or act of hostility; the first act of injury, or first act leading to a war or a controversy; unprovoked attack; assault; as, a war of aggression. "Aggressions of power." Hallam
Syn. -- Attack; offense; intrusion; provocation.
Shades of meaning
Fernald has no entry of its own for aggression; he lists it under attack.
Where he writes about it:
An attack may be by word; an aggression is always by deed. — from attack
An assault may be upon the person, an aggression is upon rights, possessions, etc. — from attack
An invasion of a nation's territories is an act of aggression; an intrusion upon a neighboring estate is a trespass. — from attack