The use of poisons to kill or weaken targeted populations
Agent Orange
Depleted Uranium munitions
Atomic Warfare
Use of a pre-emptive nuclear strike in order to destroy """strategic""" (civilian) targets
Hiroshima
Nagasaki
US has a qualified 1st strike policy, meaning that they are open to using nukes offensively, and may do so before a potential enemy has escalated the conflict to that level
Poisoning Warfare
Atomic Warfare