I'm going to assume here that you're referring to the March 1968 massacre that took place in and around the area of Sơn Mỹ village. From the outset, it's important that we recognise this for what it was: mass murder of civilians, not a battle [side note: there is of course a long, long history of referring to massacres as 'battles'. See for example the 'battle' of Wounded Knee in 1890. It was characterised as such in news reports of the time, but was simply mass murder]. I think that this is the really important lesson that comes out of your question.

Now, I'm not an expert on the post-facto legal framing of the events of March 1968 (the historian Patrick Hagopian has done work on this in his 2013 book American Immunity: War Crimes and the Limits of International Law) but these events need to be placed within the wider context of the war as a whole, and of the ongoing Tet Offensive, which had begun in January 1968. US forces were attempting to regain the initiative across southern Vietnam, and believed that locally they would be able to find the PAVN's 48th Battalion in the area of Sơn Mỹ. But as events made clear, there were no weapons or troops in Sơn Mỹ and the surrounding hamlets. The massacre achieved nothing, either strategically or tactically, other than the rape and murder of hundreds of Vietnamese civilians. And we must bear in mind that this was not simply the action of one junior officer in the form of William Calley. Such actions were encouraged from much higher up the chain of command, and were thence covered up by more senior officers.