American politicians, and Americans themselves, love to call themselves "a nation of immigrants": a place where everyone's family has, at some point, chosen to come to seek freedom or a better life. America has managed to maintain that self-image through the forced migration of millions of African slaves, restrictive immigration laws based on fears of "inferior" races, and nativist movements that encouraged immigrants to assimilate or simply leave.

But while the reality of America's immigrant heritage is more complicated than the myth, it's still a fundamental truth of the country's history. It's impossible to understand the country today without knowing who's been kept out, who's been let in, and how they've been treated once they arrive.