Many Americans simply would not exist if the country had not suffered the tragic irreplaceable genetic losses between 1861-1865. Over 600,00 killed outright and probably an equal number so damaged as never to procreate again. Many of the lineal descendants of the original Americans were obliterated. The country was forced to open the immigration gates to Eastern and Southern Europeans to restock and provide the manpower necessary for the industrial revolution. The irony is that today, in the Northeastern states the descendants of those immigrants clearly outnumber and dominate the descendants of those who served in the Civil War and had those passionate arguments. Sic transit omnia.
Rather there were no immigration barriers before the 20th century. The oldest continuously operating store in Los Angeles is a Japanese candy store, showing some degree of continuity of our immigrant community here.