Of course in history the U.S. has the dark episodes of the displacement of Natives and of the trafficking of slaves. Of course, Natives too were many times brutal with other Natives; and slaves were mostly kidnapped and sold into slavery by their neighbors. And then, it isn't like any other country on Earth is exempt of similarly negative acts.

But even not counting Natives and slave descendants; the U.S. is easily the most diverse country in the world which isn't an heir of millennia of diversity: India or Indonesia; Nigeria or the Congo; Brazil.

And as you say, it also receives more immigrants, legal and illegal, than any other country in the world; and few surpass it in percentage. Those few are Canada (which is less than 15% the population of the U.S.,) countries which receive refugees from neighbors at war (Pakistan), micro state tax havens (Monaco) and a few states which heavily rely on foreign labor both blue and white collar but where acquiring citizenship is all but impossible (the small Gulf states, Singapore). That reveals both that Americans welcome foreigners and that foreigners are comfortable looking to migrate to the U.S.