The 1965 Immigration and Nationality Act, whose 50th anniversary comes on October 3, officially committed the United States, for the first time, to accepting immigrants of all nationalities on a roughly equal basis. The law eliminated the use of national-origin quotas, under which the overwhelming majority of immigrant visas were set aside for people coming from northern and western Europe.

In the subsequent half century, the pattern of U.S. immigration changed dramatically. The share of the U.S. population born outside the country tripled and became far more diverse. Seven out of every eight immigrants in 1960 were from Europe; by 2010, nine out of ten were coming from other parts of the world. The 1965 Immigration Act was largely responsible for that shift. No law passed in the 20th century altered the country’s demographic character quite so thoroughly. But its effects were largely inadvertent. The law’s biggest impact on immigration patterns resulted from provisions meant to thwart its ability to change much at all.

The United States has long struggled to define what it really means to become American and which immigrants qualify. George Washington declared the country was open to “the oppressed and persecuted of all Nations and Religions,” but the idea persists that America is a “Judeo-Christian nation,” that being a Muslim American is a contradiction in terms, and that some nationalities are inferior to others.

Such questions should have been settled 50 years ago with the passage of the 1965 Act. For supporters, the intent of the legislation was to bring immigration policy into line with other anti-discrimination measures, not to fundamentally change the face of the nation. “We have removed all elements of second-class citizenship from our laws by the [1964] Civil Rights Act,” declared Vice President Hubert Humphrey. “We must in 1965 remove all elements in our immigration law which suggest there are second-class people.”

At the signing ceremony on Liberty Island, President Lyndon Johnson said the new law “corrects a cruel and enduring wrong in the conduct of the American nation,” but he downplayed its expected effect. “The bill that we sign today is not a revolutionary bill,” he insisted.

Opponents of the reform proposal had argued that the United States was fundamentally a European country and should stay that way. “The people of Ethiopia have the same right to come to the United States under this bill as the people from England, the people of France, the people of Germany, [and] the people of Holland,” complained Senator Sam Ervin, a Democrat from North Carolina. “With all due respect to Ethiopia,” Ervin said, “I don’t know of any contributions that Ethiopia has made to the making of America.” The critics highlighted population pressures in the developing world and predicted the United States would find itself inundated by desperate migrants from poverty-stricken countries.

Only a few supporters of the 1965 legislation said the country could and should be willing to accommodate more immigrants of color. “The American nation today stands as eloquent proof that there is no inherent contradiction between unity and diversity,” declared Representative Peter Rodino of New Jersey, a Democrat of Italian origin. The more typical response to the nativist arguments was simply to deny that the proposed immigration reform would bring any significant shift in the pattern of immigration. Secretary of State Dean Rusk, testifying in Congress, said he saw no indication of “a world situation where everybody is just straining to move to the United States.”

Such assurances did not sway conservative critics of the reform, but a last-minute change in the legislative language did alleviate their fears of a massive African and Asian influx. The original version of the 1965 Act, cosponsored by Senator Philip Hart of Michigan and Representative Emmanuel Celler of New York, both liberal Democrats, favored those immigrants whose skills were “especially advantageous” to the United States. Conservatives, led by Representative Michael Feighan, an Ohio Democrat, managed to change those priorities, giving visa preferences instead to foreigners who were seeking to join their families in the United States. Feighan, who chaired the House Immigration subcommittee, argued that a family-unification preference in immigration law would establish, in the words of a glowing profile in the American Legion magazine, “a naturally operating national-origins system,” because it would favor immigration from the northern and western European countries that at the time dominated the U.S. population.

Feighan and others were wrong. The heightened emphasis on family unification, rather than replicating the existing ethnic structure of the American population, led to the phenomenon of chain migration. The naturalization of a single immigrant from an Asian or African or Hispanic background opened the door to his or her brothers and sisters and their spouses, who in turn could sponsor their own brothers and sisters. Within a few decades, family unification had become the driving force in U.S. immigration, and it favored exactly those nationalities the critics of the 1965 Act had hoped to keep out, because those were the people most determined to move.

In the end, passage of the law did not resolve the question of America’s identity, and the debate continued. Some conservatives subsequently argued that the 1965 Immigration Act had been a scheme to curry favor with liberal special interest groups or even to establish a future demographic base for the Democratic Party. But such analyses missed the irony at the heart of a law whose most revolutionary provision was originally intended to bolster the status quo.

The debate over U.S. immigration policy has long focused on what to do about people who come to the country unlawfully. Even legal immigrants face hostility these days, though, as the prospect of a nonwhite U.S. majority prompts a revival of nativist sentiment. Xenophobia was evident as well during the debate over the 1965 act, but one difference is that the country now has 50 years of experience successfully integrating non-European immigrants. The 1965 Immigration Act has never gotten the attention it warrants as the law that finally made America the open nation it had long claimed to be. Its 50th anniversary could be an occasion for celebration.