The execution in the "infamous Vietnam execution photo" wasn't even being carried out by a US person; it was not the sort of thing I had in mind. I was thinking more of, e.g., the My Lai massacre.

Is that what you consider "extremely soft"?

The idea that the US had been "tactically winning" the Vietnam war is, so far as I can tell, far from universally held. Can you explain to me why I should believe your description rather than anyone else's?