"So the US is guilty of being racist and carrying out ethnic cleansing by displacing and exterminating the Native People?"
Has there ever been any doubt it was ?
"To judge the actions of past societies by the mores of today is a kind of simplistic "
Depends on how far into the past you go.
The Imperial system was based on the idea of "inferior" people unfit to rule themselves and superior ones who were chosen by god to lead
We were still singing hymns about when I was at school that told us to be from anywhere "from Greenland's icy mountains to India's coral Strands (anywhere but England) was to be "In error's chain"   
Jim Carroll