First, they're would be a cover up. Eventually, a whistleblower would take the story public. Then politicians would set up a commission of inquiry. It would find that proper procedures were not followed:
- Verbal orders should have been repeated back to confirm that they were heard correctly.
- Orders of any complexity at all should have been relayed or repeated in writing, with map coordinates, not just vague instructions like east or west of a river.
The colonel and the generals who gave him the orders would get quiet reprimands, and some unfortunate lieutenant with lousy lawyers would be set up as a fall guy.
Citizen activists would be outraged, and politicians would pressure the military into clarifying standard operating procedures, so that no such incident could ever happen again. And then when the standard operating procedures were eventually disregarded, it would all happen again.
(For further inspiration, do a web search for the No Gun Ri Massacre, the My Lai Massacre, etc. Or go back further into history and look up the Massacre at Béziers.)
- - -
Alternatively, under certain types of governments, the people in power would just say, "Oh, well, these things happen in war. They were just civilians anyway. Civilians are expendable, particularly if they are enemy civilians."