Final answer:
The Immigration and Nationality Act of 1965 drastically altered U.S. immigration policies, ending a quota system favoring European immigrants and significantly diversifying the American population over the following decades.
Step-by-step explanation:
When Lyndon Johnson signed the Immigration and Nationality Act of 1965 into law, contrary to his prediction, it led to a transformative shift in the demographics of the United States. This legislation, also known as the Hart-Celler Act, overturned the National Origin Formula, thereby abolishing a system that had heavily favored immigrants from northern and western Europe, and enacted preferences for family reunification and skilled immigrants. The act's passage had profound long-term effects, dramatically opening the country to immigrants from Asia, Latin America, and other regions.
Since its enactment, approximately 59 million immigrants have arrived in the U.S., significantly altering the American demographic landscape and leading to the formation of diverse communities. Despite Johnson's expectations, the act facilitated an era of substantial demographic change and had a lasting impact on the nation's immigration patterns.