Under whose command? *We* are in charge. Therefore, *we* are responsible.
The nationality of the guards makes no difference. The people who hold authority and command are the ones responsible.
If anyone can demonstrate that there was a legally constituted Iraqi governmental authority in charge during this period, then the fact that Iraqi guards (i.e., under Iraqi command) were involved is pertinent. The individuals who commit the crimes are guilty. The authority that the guilty parties represent is responsible.