This article by Dr. Walter A. Ewing provides an excellent and objective history of immigration policy in the United States since the founding of the nation. A thorough yet accessible account, this piece will give you crucial context to facilitate comprehension of the current political climate in the U.S. surrounding immigration and the evolution of immigration policy.
As you read, it is useful to make notes of things you wish to seek elaboration on, as many of these policies do not appear in mainstream textbooks and are rarely (if ever) referenced in the media!
Here is the link to the article: