This weekend marks the 50th anniversary of the Immigration and Nationality Act — the law that created the US immigration system as we know it today.
What the Immigration and Nationality Act did
The INA replaced the overtly racist immigration regime of the mid-20th century, which fully banned immigration from Asia or Africa and set strict national quotas designed to limit immigration from southern and eastern Europe. The quotas were based on the ethnic balance of the 1890 census — when, in the opinion of the Congress of the time, the United States was still a properly "white" country and wasn't in danger of being overrun with Italians and Jews.
The Immigration and Nationality Act replaced this with the legal immigration system we still use today. There's a flat cap on how many immigrants per country can immigrate each year, but individual immigrants aren't approved or denied based on where they come from. Instead, they're admitted largely through family members in the US; temporary work permits for specific employers; or refugee status or asylum (along with assorted other, smaller categories).
You can see the results in the chart above, which displays the number and origins of immigrants — naturalized citizens, legal immigrants, and unauthorized immigrants — living in the United States during the 1960 census (before the INA) and during each decade after.
In 1960, immigrants to the US were overwhelmingly European. Furthermore — at least partly because so few eastern and southern Europeans had been allowed into the country under the quota system — Jewish and Italian Americans had largely assimilated into the US, and were considered white in a way they weren't in the 1920s. But really, there were relatively few immigrants in the US at all.
The INA opened the door to millions of immigrants. The percentage of the US population that is foreign-born is almost as large today as it was during the peak of US immigration at the turn of the 20th century — when unlimited numbers of immigrants from Europe could come to the United States, but when there were only half as many people living in the US as there are today.
Changing the face of immigration to the US
What's arguably more important is that the INA changed the face of immigration to the United States. In the quota-based decades before the INA, permanent immigrants to the US were overwhelmingly white. Mexicans often came as seasonal workers (through the bracero program), but were required to go home for a portion of every year — and since it was easy to return the next year, and workers' families had to stay in Mexico, they didn't have much reason to overstay.
The INA created the large-scale Latin American migration of the late 20th century. For one thing, it allowed more Latin Americans to come to the US legally — especially Latin Americans from countries other than Mexico. But as sociologist Doug Massey recently explained in the Washington Post, it also created unauthorized immigration as we know it,
The large, seasonal bracero program was ended, and assigning an equal number of permanent immigration slots to each country meant that Mexicans who qualified to immigrate through their families had to wait for years or decades for a slot to free up. (In recent years, India, China, and the Philippines have developed the same problem: Enough immigrants from those countries have settled down, gotten green cards or citizenship, and petitioned for their relatives to join them that they've run up against the per-country quota.) Combine that with increased border enforcement during the end of the 20th century and beginning of the 21st that made it much more dangerous to slip back and forth across the border for seasonal work, and you get the current situation: a population of 11 million full-time, long-settled unauthorized immigrants, most (though not all) of whom come from Mexico and the rest of Latin America.
Meanwhile, the INA allowed widespread African and (especially) Asian migration of a type the US hadn't really ever seen before. (Previous migration from Africa had been, ahem, entirely forced.) And as you can see from the chart, while the growth of Latin American immigration to the US has slowed — overwhelmingly due to the fact that very few unauthorized immigrants have come into the US since 2007 — the number of Asian-American immigrants keeps growing at a steady clip. According to census data, more people moved from China to the US in 2014 than from Mexico. And the Pew Research Center projects that sometime before 2055, Asians will surpass Latin Americans as America's largest immigrant group.