The political answer

It’s been 100 years since the Emergency Quota Act transformed the USA into the most powerful nation on Earth by limiting immigration and restricting it to compatible nations:

The Emergency Quota Act, also known as the Emergency Immigration Act of 1921, the Immigration Restriction Act of 1921, the Per Centum Law, and the Johnson Quota Act (ch. 8, 42 Stat. 5 of May 19, 1921) restricted immigration into the United States. Although intended as temporary legislation, the Act “proved in the long run the most important turning-point in American immigration policy” because it added two new features to American immigration law: numerical limits on immigration and the use of a quota system for establishing those limits. These limits came to be known as the National Origins Formula.

The Emergency Quota Act restricted the number of immigrants admitted from any country annually to 3{cc08d85cfa54367952ab9c6bd910a003a6c2c0c101231e44cdffb103f39b73a6} of the number of residents from that same country living in the United States as of the U.S. Census of 1910. This meant that people from northern European countries had a higher quota and were more likely to be admitted to the U.S. than people from eastern Europe, southern Europe, or other, non-European countries. Professionals were to be admitted without regard to their country of origin. The Act set no limits on immigration from Latin America. The act did not apply to countries with bilateral agreements with the US, or to Asian countries listed in the Immigration Act of 1917, known as the Asiatic Barred Zone Act. However, the Act was not seen as restrictive enough since millions of immigrants from eastern and southern Europe had come into the USA since 1890. The Immigration Act of 1924 reduced the Quota to 2{cc08d85cfa54367952ab9c6bd910a003a6c2c0c101231e44cdffb103f39b73a6} per the Census of 1890 when a fairly small percentage of the population was from the regions regarded as less than desirable.

Based on that formula, the number of new immigrants admitted fell from 805,228 in 1920 to 309,556 in 1921-22. The average annual inflow of immigrants prior to 1921 was 175,983 from Northern and Western Europe, and 685,531 from other countries, principally Southern and Eastern Europe. In 1921, there was a drastic reduction in immigration levels from other countries, principally Southern and Eastern Europe.

Following the end of World War I, both Europe and the United States were experiencing economic and social upheaval. In Europe, the destruction of the war, the Russian Revolution, and the dissolutions of both the Austro-Hungarian and Ottoman empires led to greater immigration to the United States; while in the United States, an economic downturn following post-war demobilization increased unemployment. The combination of increased immigration from Europe at the time of higher American unemployment strengthened the anti-immigrant movement.

The act, sponsored by Rep. Albert Johnson (R-Washington), was passed without a recorded vote in the U.S. House of Representatives and by a vote of 90-2-4 in the U.S. Senate.

100 years later, it is clear that if the USA is going to survive another decade or two intact – which it almost certainly isn’t – a new Emergency Quota Act is required, albeit one that restores the 1910 demographics. Since this is a political solution, and since four years of the Trump administration didn’t even manage to seriously slow the invasive flow from around the world, we can safely discount any chance of this proven measure being adopted in any meaningful timeframe.

The much more likely alternative is that those demographics are more or less restored in a rump state after a considerable amount of war and ethnic cleansing over the territory of the former empire. Unless, of course, China manages to establish a Chung Kuo-like hegemony over North America, which at this point is probably more likely that the sensible solution.