I doubted your comment, then I looked it up, sure enough, according to Wikipedia Powell's handling of the assignment was later characterized by some observers as "whitewashing" the atrocities of My Lai.

http://en.wikipedia.org/wiki/My_Lai_massacre

Whether or not that was why he was promoted, I don't know. It may have had a part in it. From reading more, it seems like he's a Macnamara type who thinks some evil is done in the name of good.