The share of the U.S. population made up by immigrants has returned to the levels at the turn of the 20th century — although the makeup of today's immigrant population looks very different.

Why it matters: As we saw a century ago, and are witnessing again now, immigration brings needed labor and economic benefits, but is often met with backlash from those who fear the America they know is slipping away.

America's immigrant population has ebbed and flowed over the decades as discriminatory policies have been instituted and repealed, international crises have arisen and faded, and the economy has boomed and faltered.

Two key policies:

The 1924 national origin quota meant few non-Europeans could come to the U.S. Coupled with the Great Depression and the lead-up to World War II, this contributed to the sharp declines seen from the early 1900s until around 1965, Jeanne Batalova, a senior policy analyst at the Migration Policy Institute, told Axios.

meant few non-Europeans could come to the U.S. Coupled with the Great Depression and the lead-up to World War II, this contributed to the sharp declines seen from the early 1900s until around 1965, Jeanne Batalova, a senior policy analyst at the Migration Policy Institute, told Axios. The quotas were lifted with the 1965 Immigration and Naturalization Act — which is largely still the law of the land. This came during the civil rights movement and the Cold War to win over African and Asian nations to capitalism and democracy, according to Batalova.

Where immigrants have come from:

For most of the 19th and 20th centuries, the vast majority of immigrants came from Western and Northern Europe. The Irish fled famine, the Germans fled political instability, and Italians primarily wanted better economic opportunity.

the vast majority of immigrants came from Western and Northern Europe. The Irish fled famine, the Germans fled political instability, and Italians primarily wanted better economic opportunity. The 1965 act ended a program that allowed Mexicans to work on U.S. farms but remain residents of Mexico. That changed the nature of immigration from Mexico and from Central America "to primarily unauthorized," Batalova says.

that allowed Mexicans to work on U.S. farms but remain residents of Mexico. That changed the nature of immigration from Mexico and from Central America "to primarily unauthorized," Batalova says. In 1986, the U.S. gave legal status to almost 3 million undocumented immigrants — an overwhelming majority of them from Mexico. These new green card holders could then sponsor additional family members.

— an overwhelming majority of them from Mexico. These new green card holders could then sponsor additional family members.

There has been a surge of Central American asylum seekers in the U.S. over the past several years as political chaos, poverty and violence have ravaged many of those nations.

of Central American asylum seekers in the U.S. over the past several years as political chaos, poverty and violence have ravaged many of those nations. New laws also opened the door to immigration from Asia — initially from India and Taiwan, and later China.

— initially from India and Taiwan, and later China. Following the Vietnam War, there was an influx of Vietnamese people and other citizens of the region who fled to the U.S. as refugees.

there was an influx of Vietnamese people and other citizens of the region who fled to the U.S. as refugees. Most recently, there's been a wave of immigration from African countries that began in the 1990s and 2000s for a wide variety of economic, political and humanitarian reasons.

The big picture: Nationalist phobias prompted the original immigrant quotas in the U.S., according to Guillermo Cantor, research director at the American Immigration Council. There were fears that the number of Chinese coming to the U.S. for work would change the culture, or that German would become the dominant language in Pennsylvania. There are echoes of those sentiments in the current political climate.

What's next: Congress has been slow to make major changes to immigration laws, and many of the Trump administration's efforts have been held up in courts, making it difficult to predict just yet whether current trends will be reversed, immigration experts tell Axios.

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