People coming over the Mexican border and from Muslim countries bear the most scrutiny now, but it was once somebody else: Germans, Irish, Chinese people. We are a nation of immigrants, but also a nation that occasionally and vociferally opposes the immigration of certain groups of people.
Erika Lee, historian and professor at the University of Minnesota, lays out the full story, from colonial times until now, in her book America for Americans: A History of Xenophobia. The author visits to explain the durability of xenophobia, and the risks it presents to the country.