Sunil is not talking about invasions... but the annihilation of the existing tribes like the lakotas...
America is a country formed by erasing natives and forging "the white man" alliance. Remember KKK in glorious America?, which was hell bent on killing negroes...
American history is not known until a small group of british started settlements...The incas, mayans have become extinct...
So the invasions you read in in medieval history would definitely be about European, Arab and Asian ones...!!!
