The Civil Rights Movement was America’s greatest moral crusade, the culmination of years of determined citizen activism, leading  to radical changes in the 1960s. Or so we thought.

In the space of 10 years Congress passed four major civil rights acts, the Voting Rights Act and the Fair Housing Act.

And we didn’t just settle for a level playing field. We embraced affirmative action, expanded anti-poverty programs, rewrote  history books, renamed public buildings, erected new statues, and promoted talented Black people in journalism, sports and Hollywood. Multiple Black mayors, city councilors and police chiefs were elected.

Black incomes grew after World War I, and the Black-white wage gap narrowed until around 1970. This remarkable achievement was mostly accomplished in the face of Jim Crow laws, and with minimal help from white people.

Following the Civil Rights Movement, we expected this gap to disappear. In fact, in the ensuing half century, the gap widened to 1950 levels. What happened?

A new  book, “Back of the Hiring Line” by Roy Beck, offers a meticulously documented perspective on this national puzzle. Along with copious employment data and multiple immigration and labor historians, Beck tells the  story through the writings and speeches of prominent Black leaders such as Booker T. Washington, Frederick Douglass, W.E.B. DuBois, A. Phillip Randolph, and Black-owned newspapers, all of whom doggedly advocated for tight labor policies following the Civil War.

Finally, a mounting demand by Black and labor activists forced Congress to cut immigration numbers in half in 1924. The Great Wave ended. Blacks began migrating North to fill blue collar jobs in factories deprived of  European workers. Blacks got their first shot at better paid jobs in manufacturing.

During this half century of tight labor, wages increased, wealth disparities shrank, the middle class exploded, and Black average incomes skyrocketed. Everyone benefited. Employers were forced to hire and train from within, and negotiate with unions for pensions, cost of living wages, and health care.

But, in 1965, Congress changed course again, radically expanding annual immigration numbers. Since 1990 Congress, placating the tech lobby, has given away millions of high-paying tech jobs through guest worker programs. The demand for foreign workers is unending. This February, the House passed — along party lines — the America Competes Act, expanding yet again foreign worker visas.

With the surge in foreign workers, some employers tended to prefer immigrants who were willing to accept long hours and lower wages. Blacks were gypped in one industry after another where previously employed. They also missed out on entry level jobs in technology, losing the opportunity to develop those skills, networks and work experience that lead to upward mobility and generational wealth.

Blacks earn one in 10 computer science degrees nationwide, but account for only 2.6% of Silicon Valley tech workers. Seventy-one percent of tech workers in Silicon Valley today are foreign born. The employment/opportunity ladder that propelled many immigrants was denied to Blacks.

And the results of shrinking work availability on vulnerable populations are evident in the crime, welfare dependence and family dissolution of our inner cities.

In the 1990s, Congress authorized the formation of the U.S. Commission on Immigration Reform. After seven years of study, the commission told Congress to significantly decrease immigration, setting an annual cap of 550,000. The current level of immigration is over one million annually. Congress  ignored the experts they had chosen to consult, and set immigration numbers to please business lobbies wanting more workers and more consumers. And the clamor for more foreign labor continues.

Beck blames Congress, not immigrants, and he doesn’t claim that all problems facing Black communities are driven by immigration. Many Blacks have made substantial progress. But a disproportionate subset is stuck in generational poverty.

Immigration numbers have played a largely unrecognized role in forcing vulnerable groups to compete against each other, and Blacks have been especially harmed.

The solution is simple: Congress should reduce the numbers. All workers, immigrant and native born, would benefit from tighter labor markets and higher wages.

Jonette Christian of Holden is a founder of Mainers for Sensible Immigration Policy.