I will trace it to the Immigration and Nationality Act of 1965.
While I mostly agree with you on immigration, that problem largely stems from abortion. We have murdered 50 million of our own countrymen and they must be replaced with someone.
Read up on _birth_ratios_ and see that USA has fallen _well_ below the minimum sustainability level of 2.2 (or what it is [not looking it up right now]).
They are intertwined...