Dude, this happened in living memory. Americans are completely open about it.
Operation Ranch Hand had two primary objectives: (1) defoliation of trees and plants to improve visibility for military operations, and (2) destruction of essential enemy food supplies. Source
Attacking civilian food sources is a war crime. Source
It's all a bit moot, since the USA gives zero shits about the UN, ICC and when they can hide it, the Geneva Convention. Hell, the ICC list of War Crimes is like a runsheet of what the Americans got up to in Vietnam. And we haven't even discussed Napalm.
Have they improved? Yeah, probably. But then you watch the Collateral Murder video, and question that.
I haven't equated the Americans with ISIS. But I can understand why an Iraqi would and would give their position serious consideration.