It's not particularly the USA, Doug. This is the kind of thing that happens in military occupations, both the killings and the cover-ups. There are plenty of comparable instances in the history of all empires and occupations - British, French, Russian, Israeli, Iraqi... And in the history of the manifest destiny continental empire that became the USA. Sometimes the regimes involved, and their apologists, are left wing, sometimes they are right wing.

And sometimes the harshest critics of atrocities are people who might be assumed to be natural apologists for them. For example when Enoch Powell denounced the Conservative Government of which he was a member for trying to cover-up atrocities in the Hola Camp in Kenya, where detainees were beaten to death. "Let justice be done though the heavens fall" ("fiat justicia ruat coelum") is not an especially left liberal sentiment, it is a principle that has guided decent people of all political tendencies in these kinds of situations.

Whatever may be the case about the actual killings, the military cover-up isn't at this time a matter of allegations. The initial accounts of the killings, sustained for a number of months till Time Magazine broke the story back in March, have been confirmed as having been completely false.

In some ways the cover-up is in the central issue, more especially in the light of the fact that the responsibility for investigating both the killings and the cover-up are the military. When atrocities are brought to light the effect is likely to be to reduce the likelihood of more atrocities, at least in the short run; when cover-ups succeed, that increases the likelihood of more stuff that need to be covered up.

And it's worth remembering that in the case of My Lai, where the cover-up was successful for quite a bit longer than has been the case with the Haditha alleagtions, no one appears to have been punished for their part in it, or to have had their career damaged. Including the young Colin Powell. (And the same has been true in the British context in regard to Bloody Sunday.)