Martha S. Jones is the SOBA presidential professor of history at Johns Hopkins University and a co-president of the Berkshire Conference of Women Historians. She is the author of the forthcoming book "Birthright Citizens: A History of Race and Rights in Antebellum America."

The amendment that once extended citizenship to black Americans now creates obstacles for undocumented immigrants. (Mary Altaffer/AP)

One-hundred-fifty years ago, the United States was hard at work defining citizenship. It was an effort to fix one of the central oversights of the Constitution: Who counts as a citizen is fundamental to a democracy, yet the nation’s founding document was silent on the subject. Some argued that the framers intended citizenship only for white Americans, others that free black Americans must be treated as citizens, as well. Absent an express constitutional provision, a debate grew up around the relationship between race and citizenship. In the wake of a Civil War that abolished slavery, this question demanded resolution.

Before the war, no community had been more haunted by the Constitution’s absence of a citizenship clause than freed slaves and their descendants. They had lived and labored for generations in a legal limbo that the new 14th Amendment’s birthright citizenship clause promised to resolve. In July 1868, when Secretary of State William Seward ratified the amendment, black Americans, along with all those born in the United States, were declared citizens.

There is much to admire in the work of Reconstruction-era lawmakers who took this step toward a more complete democracy. But today, their definition of what makes an American falls short of resolving our citizenship crisis. Unauthorized immigrants and their communities find too little recourse in the birthright principle. Despite building families and institutions across generations, they are, not unlike former slaves, excluded from the nation’s borders of belonging.

These two communities — former slaves and unauthorized immigrants — might appear far apart in time and circumstance. Free African Americans had origins in a system of captivity, brutal compulsion and forced labor, while unauthorized migrants are here by way of economic or political coercion and work as free laborers.

Still, their experiences converge when it comes to citizenship. In both instances, the nation permitted people to establish thriving communities without resolving their legal status. This history and two enduring aspirations link former slaves and unauthorized immigrants across 150 years: The desire to remain within the nation’s territory and to defend against the constant threat that, at any moment, officials might arbitrarily remove them beyond its borders.

Former slaves and their descendants faced grave threats as noncitizens. In the decade before the Civil War, free African Americans numbered nearly 500,000, or 1.6 percent of the nation’s population. Most had lived in the United States for generations, building families, schools and churches. Their labor was the engine of prosperity, whether on rural farmlands or urban cityscapes.

Still, free black Americans lived under constant pressure to leave the country. So-called black laws limited their travel, work, and public gatherings. Colonization societies organized to send them to Africa, the Caribbean or Canada. In 1857, the Supreme Court ruled in Dred Scott v. Sandford that black people were not citizens of the United States, barring them from suing in federal courts. These forces all aimed to remove African Americans from the nation, whether by coercion or force.

A lack of citizenship defined a decades-long reign of terror that reached into daily life. If they were not citizens, could black Americans own homes and businesses? Could they worship freely and govern their own churches? Could they control the custody and education of their children? Could they travel to visit family or secure jobs? Could they use the courts to protect their wages or their personal safety? No authority would say for certain where these noncitizens stood.

The answers came about only through a revolution. Four years of civil war and the near dissolution of the nation brought an end to slavery and the enactment of a new constitutional regime. The adoption of the 14th Amendment in 1868 guaranteed citizenship to those born or naturalized in the United States, including former slaves. Black Americans would face subsequent challenges to their civil and political rights, but the 14th Amendment ensured that they would never again face the threat of removal.

For today’s unauthorized immigrants, however, the 14th Amendment provides no relief. The once-radical promise of birthright citizenship is beyond their reach. Their lives tell part of the story. The Pew Research Center estimated that in 2015, 11 million unauthorized immigrants — 3.4 percent of the overall population — lived in the United States. These men, women and children have built families, homes and businesses across generations. Their labors drive economic growth, especially in agriculture, industry and the service sector.

Integral to the country in which they live, today’s unauthorized immigrants nonetheless have no access to the rights of citizens. They have no unassailable claim to place. This year began with anti-immigrant threats of mass deportation. What followed has been more quotidian but no less harrowing. Workplace raids by Immigration and Customs Enforcement discourage even documented immigrants from their jobs. Public gatherings are canceled because community celebrations invite surveillance and detention.

With no path to citizenship for unauthorized immigrants, the 14th Amendment instead becomes a generational scythe. Families of mixed status confront dislocation and separation as birthright slices through households, severing cross-generational ties. It is a haunted existence that produces lives subject to the arbitrary winds of politics and policy.

For communities that include unauthorized immigrants, a new reign of terror is underway. As in 1867, people living in the United States today face schemes that aim to forcibly excise them from the nation.

And so we are confronted with the limits of the 14th Amendment in another chapter of America’s longer struggle to define who has access to the rights and privileges of belonging. The 19th-century solution, citizenship by birth, no longer reaches far enough.

Ambiguities about the belonging of unauthorized immigrants, who have no clear route to citizenship, represent a new challenge to our democratic ideals: How will we create a more expansive definition of citizenship that reaches all those whose labor and lives contribute to the prosperity and strength of the nation — and whose children are, thanks to the 14th Amendment, citizens in their own right. Until we resolve this dilemma, we will continue to watch as a cramped definition of citizenship makes outlaws of friends, neighbors, co-workers and family members.

The 14th Amendment — passed on this day nearly 150 years ago — resolved the most pressing citizenship question of the 19th century. It was an instrument of hope as birthright extinguished threats of removal and opened doors to membership in the body politic for former slaves. Nearly a century and half later, a new citizenship crisis exposes the Amendment’s unintended but tragic limits and calls out for solutions as bold and imaginative as the 14th Amendment once was.