Our knowledge of the past is like memory - we don't actually remember what it was like when we were 3 - we remember remembering what it was like when we were 3.

It's filtered through secondary sources that are never completely reliable, and there is information lost because people die without telling anyone what they saw or did, or there's confusion, or, in war, the winner's perspective is presented as the only facts.

In the US, the obvious example is the history of the west, and the natives of North America. It was once viewed as the bringing of civilization to savages for their own good. Now its viewed as genocide. Because its being viewed from a different perspective.