Colin Woodard is a journalist and the author of five books, including “American Nations: A History of the Eleven Rival Regional Cultures of North America” and the forthcoming “American Character: A History of the Epic Struggle Between Individual Liberty and the Common Good.”

The Kim family’s path to America began with 14-year-old Nam Soo’s harrowing escape from the North Korean army’s advance on Seoul in January 1951. She and her mother were lucky enough to find a place on the roof of the last freight train out of the city and somehow survived the three-day trip to safety as others froze or fell to their deaths around them. Her mother, exhausted by the journey, died a few months later, but Nam Soo survived the war and — via Saigon, Australia and a brother who had served with the U.S. military — finally arrived in the United States in 1980 with her husband and children, whose adventure was only beginning.

Theirs is one of the powerful human stories captured by veteran NPR reporter Tom Gjelten in “A Nation of Nations,” a new book exploring the staggering impact of the 1965 Immigration Reform Act on the United States in general and Fairfax County, Va., in particular. From a village in rural Bolivia, a farm in war-torn El Salvador and a dissident household in Libya, his central subjects struggled to get to the county in the 1970s and ’80s, then to build careers, businesses and better futures for their children. In the process they helped transform Fairfax from a racially segregated Southern county represented in Congress by white supremacists to one of the wealthiest and most culturally diverse counties in the United States, a national symbol of contemporary “edge gateway” immigration, in which newcomers concentrate on the outskirts of a city rather than in its urban core.

None of it would have happened were it not for the passage of the 1965 immigration act, the consequences of which are Gjelten’s central concern. Signed into law 50 years ago at the height of Lyndon Johnson’s Great Society, the act ended the 41-year-old practice of assigning immigration quotas based on ethno-national origins; these were in place to preserve the allegedly Northern European character of the United States. No longer would immigration policy be based on what Sen. Ellison Smith (D-S.C.) had called “the preservation of that splendid stock,” the “pure, unadulterated Anglo-Saxon stock,” and on the restriction of supposedly inferior Eastern and Southern Europeans, Africans, Arabs and especially Asians, who were explicitly barred from citizenship.

“This system violated the basic principle of American democracy — the principle that values and rewards each man on the basis of his merit as a man,” Johnson said at a signing ceremony at the foot of the Statue of Liberty. “It has been un-American in the highest sense, because it has been untrue to the faith that brought thousands to these shores before we were a country.”

‘A Nation of Nations: A Great American Immigration Story’ by Tom Gjelten (Simon & Schuster)

Surprisingly, few in Washington fully appreciated the consequences of their reforms, which the Johnson administration claimed would not result in major changes in immigrants’ numbers or national origins. Their main congressional adversary, Rep. Michael Feighan (D-Ohio), had exacted a concession he thought would preserve the racial and ethnic status quo: an insistence that the number of slots reserved for high-skill applicants be reduced and those for family unification purposes increased, with nearly a quarter set aside just for the siblings of U.S. citizens. Because there were then so few Asian-, African- or Arab-born citizens, he reasoned, the stipulations would act as “a naturally operating” national quota system.

“None of the people involved in the 1965 reform of U.S. immigration policy understood what they were doing,” Gjelten observes. Feighan’s plan produced exactly the opposite of what he intended: The European share of immigrants fell from 7 out of 8 in 1960 to 1 in 10 in 2010, a consequence of improved economic prospects in Europe, better transportation and communication links with other parts of the world and the act’s liberal family unification provisions. “A doctor or an engineer arriving from India, a technical worker from South Korea, a student from Africa who found employment in the United States, or a refugee from Afghanistan provided an entrée for an entire family network.”

Fifty years on, the result has been a far more diverse and decidedly less Anglo-Saxon country, and a reassessment of what it means to be American. The late Samuel Huntington’s contention that we’re, culturally speaking, an Anglo-Protestant nation may have once made sense in New England or to the gentleman planters of the lower Chesapeake, but it was never true of great swaths of the country where the founding colonists were Dutch or German and the settlement ideal was cultural pluralism. That 350-year-old debate — are we an assimilative melting pot or a multicultural mosaic? — gains poignancy in a world where newcomers are more diverse than ever and can often maintain close contact with their nations of origin.

Fairfax County makes for a fascinating case study, not least because of its Dixie heritage. Its racial caste system was largely still in place in 1965, but by the time new immigrants began arriving in large numbers, African Americans had won hard-fought battles for school integration and to have paved roads and sidewalks in their neighborhoods. “We were finally getting accepted, and all of a sudden here comes another group, and they qualify as a minority,” one African American former PTA president told Gjelten, who ably describes the inevitable tensions that followed as social expenditures were diverted to help the newcomers.

Gjelten sets up the key national policy debates. These include concerns about a breakdown in assimilation, focused especially on Muslim immigrants after the Tsarnaev brothers’ attack on the Boston Marathon, and the emergence of Anwar al-Awlaki, the Falls Church, Va., imam and eventual al-Qaeda leader who was killed by a drone strike in Yemen in 2011. Gjelten also discusses the debate over continuing to prioritize the immigration of brothers and sisters of U.S. residents over that of individuals with skills and expertise in short supply in the economy. Some readers may be disappointed he doesn’t explore the latter debate more deeply by, say, examining the merits and demerits of the skills-based Canadian immigration model.

That said, Gjelten has produced a compelling and informative account of the impact of the 1965 reforms, one that is indispensable reading at a time when anti-immigrant demagoguery has again found its way onto the main stage of political discourse.

A Nation of Nations
A Great American Immigration Story

By Tom Gjelten

Simon & Schuster.
405 pp. $28