Final answer:
The fundamental source of U.S. Immigration Law is the Immigration and Nationality Act (INA) of 1965, which ended the previous National Origin Formula and allowed immigrants from diverse global regions to enter the U.S., significantly altering America's demographic composition.
Step-by-step explanation:
The fundamental source of United States Immigration Law is the Immigration and Nationality Act (INA). The INA, passed in 1965, replaced the National Origin Formula that had been in place since 1921. This discriminatory system limited immigration primarily to those from northern European countries based on race and ethnicity, but the INA opened doors for immigrants from Latin America, Asia, Africa, and the Middle East, significantly diversifying the American demographic.
Since the passage of the INA, approximately 59 million immigrants have come to the U.S., forever altering the country's population mix. The INA sets forth the legal foundations for immigration, including the requirements for employers to verify citizenship or legal immigration status, thereby prohibiting the hiring of illegal immigrants and protecting the rights of legal immigrants.
To gain a broader understanding of U.S. immigration laws throughout history, resources such as the Migration Policy Institute offer comprehensive timelines detailing how naturalization and immigration legislation have evolved from 1790 to the present day.