There's a documentary from 1972 called Winter Soldier, where veterans of the US invasion of Vietnam testified to their war crimes. Here are a few excerpts of the transcript: http://links.org.au/node/3343

Consider the following recollection of Vietnam-style “counter-insurgency” warfare, provided by Scott Camil, a former member of the 1st Marines:

Anybody that was dead was considered a VC. If you killed someone they said, "How do you know he's a VC?" and the general reply would be, "He's dead," and that was sufficient. When we went through the villages and searched people the women would have all their clothes taken off and the men would use their penises to probe them to make sure they didn't have anything hidden anywhere and this wasremoved but it was done as searching… The main thing was that if an operation was covered by the press there were certain things we weren't supposed to do, but if there was no press there, it was okay. I saw one case where a woman was shot by a sniper, one of our snipers. When we got up to her she was asking for water. And the Lt. said to kill her. So he ripped off her clothes, they stabbed her in both breasts, they spread-eagled her and shoved an E- tool up her vagina, an entrenching tool, and she was still asking for water. And then they took that out and they used a tree limb and then she was shot.

An ex-machine gunner with the 1st Air Cavalry detailed the routine violence that accompanied cargo runs on his CH-47 “Chinook” helicopter:

It was quite usual that there would be a sniper outside a village in the foliage, in the trees, and if we took fire from one sniper we'd return fire on that sniper and then continue to spray the entire village with machine gun fire and M-16 ammunition until we either ran out of ammunition or we had flown so far away from the village that we could no longer reach them with the weapons…The free fire zones were posted on the operation map in the operations tent and this gave us a policy to kill anything that moved within that area.

Sadistic games at the expense of civilians were used to spice up the day:

Rotor wash was also used to blow down the huts, literally blow down the villages….So we'd come in and flair on a ship and just blow away a person's house. Also, the Vietnamese, when they've harvested a crop of rice, put it out on these large pans to dry and that harvest is what is supposed to maintain them for that season— what they're supposed to live on. We'd come in to flair the ship, and let the rotor wash blow the rice, blow their entire supply of food for that harvest over a large area. And then laugh, as we'd watch them running around trying to pick up individual pieces of rice out of a rice paddy.

While it was unusual for hundreds to be gunned down in a single location (as occurred infamously at My Lai in April 1968), the Winter Soldier testimony confirms that it was nothing out of the ordinary for dozens or scores of civilians to be slaughtered in “search and destroy” missions:

We moved into a small hamlet, 19 women and children were rounded up as VCS— Viet Cong Suspects— and the lieutenant that rounded them up called the captain on the radio and he asked what should be done with them. The captain simply repeated the order that came down from the colonel that morning. The order that came down from the colonel that morning was to kill anything that moves, which you can take anyway you want to take it… I turned, and I looked in the area. I looked toward where the supposed VCS were, and two men were leading a young girl, approximately 19 years old, very pretty, out of a hootch. She had no clothes on so I assumed she had beenremovedd, which was pretty SOP [Standard Operating Procedure], and she was thrown onto the pile of the 19 women and children, and five men, around the circle, opened up on full automatic with their M-16s. And that was the end of that.