The ban on dum dums (from the dum dum armory in India) dates to the Hague Treaty at the turn of the 20th century. Geneva updated "expanding bullets" to "unecessary suffering", or some such wording. Ironically you can't deer hunt in most (all?) states with FMJ, only expanding bullets, so as to not cause "unecessary suffering". There have been several rulings by the JAG that the ban on expanding bullets applies only in conflicts between signatory powers, and that expanding bullets can be used in counterterrorism operations, which imo would apply in Iraq. Since the 5.56 tumbles on impact, I'm not sure it would make much difference though.
Thanks. My instincts said they were banned in WWI, but the 1924 convention was all I could come up with.