" The immigration restriction regime begun in 1924 hardened racial lines, institutionalized new forms of race-based discrimination, codified the fiction of a “white race,” and introduced a new legal category into American life: the “illegal alien.” Europeans, deemed “white,” classified into their national origins, and ranked according to their desirability, could immigrate in limited numbers; entering the United States as legal aliens, they could become naturalized citizens. Chinese, Japanese, Indians, and other Asians, deemed nonwhite, could not immigrate into the United States legally, were deemed unassimilable, and were excluded from citizenship on racial grounds. More profoundly, the law categorized Europeans as belonging to nations—they were sorted by “national origin”—but categorized non-Europeans as belonging to “races”—they were sorted into five “colored races” (black, mulatto, Chinese, Japanese, and Indian). "

― Jill Lepore , These Truths: A History of the United States