Final answer:
The United States responded to mass migration of immigrants with laws and policies aimed at restricting immigration.
Step-by-step explanation:
The reaction of the United States to the mass migration of immigrants varied throughout history. In the early 1900s, the United States passed the Emergency Quota Act of 1921 and the Immigration Act of 1924, which aimed to restrict immigration, particularly from Southern and Eastern Europe. These laws were a response to the fear of increasing immigration and the rise of nativism. The United States also implemented policies such as the construction of a border fence along the U.S.-Mexico border in the 2000s.