In the 1970s, I clipped out a cartoon that showed archaeologists digging out skeletons pierced by axes and spears. One said, "3.75 million years old - and they're definitely human." (Yes, I do remember such trivia very clearly.) While this site from Germany dates only to 1200 BC, it's important for containing only human and horse remains and weapons. In other words, it's the site of a battle with at least 100 participants.
COMMENT: I'm not sure why this one caught my eye. We knew people were fighting long before this. But there's a difference between knowing there were battles and actually getting a picture of how human-on-human violence (sadly) evolved.