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."

Jacksonville Jaguars players kneel before the national anthem before a game against the New York Jets in October 2017. (Reuters)

Thursday morning, President Trump weighed in on a new NFL rule that will fine teams if players on the field protest during the national anthem. “You have to stand proudly for the national anthem, or you shouldn’t be there,” he said, before adding: “Maybe you shouldn’t be in the country.”

Had the president just threatened a group of predominantly black Americans with deportation? It’s tempting to dismiss such a remark as mere bluster from a man well-known for his political hyperbole. But there was just enough truth — historical truth — in the president’s quip that it should not be left to dissolve into the ether. Black Americans have faced such threats before, and those threats were never idle.

It has never been easy to be free, black and deemed disruptive in the United States. The vilification of this status has deep roots in our national consciousness that date to the emergence of the first free black communities during the post-Revolutionary era. In the wake of the Revolution, the number of free black people — many of them former slaves — grew. Individual slaveholders responded to the principle of liberty that independence embodied, and Northern states began to abolish slavery, some immediately and others gradually.

Liberation raised a pressing question: Where did free black people fit in the new nation? Some speculated that they were a disruptive force, whether as an unproductive burden on the state or as a thriving and provocative example to those still held in bondage. Leading figures like Thomas Jefferson observed that slavery would eventually end. Yet they envisioned no future for an interracial republic in the United States. No, they had another remedy in mind: colonization.

Colonization meant removal. Not forced removal but, rather, a push for “self-deportation.” Political leaders argued that former slaves — free people, many of whom were asserting claims as members of the body politic — should be persuaded to leave the United States. “Persuasion” was a slippery term that meant nobody anticipated removing them by physical force. Instead, colonizationists proposed using enticements, support and even flattery to relocate black Americans beyond the nation’s borders.

The American Colonization Society was the 19th century’s most popular political movement. Founded in 1816, it transcended the differences between political parties and regions. Its membership dwarfed that of the radical anti-slavery movement that was gaining traction by the 1820s. The society’s tactics included the establishment of West African colonies such as Liberia, the outfitting of ships to transport migrants and the allocation of provisions that would enable new settlers to start over in places far from home.

Colonization relied upon sticks as well as carrots. “Black laws” began appearing on the books in many states. They regulated daily life: travel, work, commerce, public gatherings, access to courts and the ownership of guns and dogs. Cumulatively, these laws, colonizationists hoped, would pressure free black Americans to leave the country. The strain of living under close scrutiny, with their economic and political rights circumscribed, might make the prospect of emigration just tempting enough.

Some states even considered radical colonization schemes that provided for the forced removal of free black people from the country. It might sound far-fetched to propose physically relocating hundreds of thousands of people against their will. But African Americans observed the example of Indian Removal and the brutal Trail of Tears and believed that the United States was prepared to force their exile, as well.

They resisted colonization — by carrot, stick or force. They organized, raised funds and gave birth to a radical abolitionist movement that opposed slavery by any and every means necessary. Yes, some did give in to the temptations of colonization. Such migrants succumbed to the view that there was no future for them as equal citizens in the United States. Perhaps it was a white man’s country after all.

It was the 14th Amendment’s birthright citizenship provision that finally put an end to the threat of colonization. Finally recognized as citizens, former slaves — now numbering in the millions because of slavery’s abolition — could count upon one right that all citizens enjoyed: the right to occupy the territory. Their civil and political rights would be debated for many decades to come. But it was immediately certain that black Americans would never again be subjected to wholesale removal schemes, even as they might continue to claim rights and be a “disruptive” presence.

If the 14th Amendment dashed the hopes of colonizationists 150 years ago, why then should we take seriously the president’s threat that NFL protesters “should not be in the country?” In part because this is the same president who has repeatedly called for the repeal of the very same 14th Amendment and its birthright citizenship provision. On the stump, candidate Trump proposed that ending birthright would discourage the entry of unauthorized immigrants and undercut the claims of their children born on U.S. soil.

But threats to undercut or change the terms of citizenship extend much further than to immigrants. Perhaps black American dissidents will also be among those deemed undesirable and disruptive such that a new citizenship regime would make them — after 150 years of birthright protection — again subject to removal without cause and without due process of law.

Without the security that citizenship affords, exercising fundamental rights becomes a dangerous proposition.

In the 19th century, black men and women often did not know whether the convening of a church congregation might lead to arrest. They were vulnerable to detention and fines if they traveled between states. They dared not speak against the institution of slavery in many places, for fear that they would be prosecuted and sold into slavery. Might black Americans today face the same hard choice: suppressing their own rights to avoid a worse fate? When protests like taking a knee during the national anthem can lead the president to threaten deportation, the 21st-century lines that govern race, rights and citizenship have become dangerously entangled.

NFL players do not stand in the same shoes as did former slaves. They have access to vast resources, to lawyers and, at least today, to a Constitution that reduces the president’s musing to overstatement. In the coming months, as a new season approaches, the influence of owners, coaches and fans will ensure that the debate over how political protests fit with professional sports will continue.

Still, it would be a mistake to dismiss such remarks as mere bluster. A history stands behind the view that those who are free, black and disruptive should be removed from the nation. For nearly a century, between the American Revolution and the Civil War, former slaves and their descendants lived under a legal regime that made their claim to U.S. soil a political football. While only a few thousand gave in to the pressures applied by those who dreamed of a white-only America, thousands more felt the chilling effect, afraid to exercise the rights guaranteed to all citizens, including that of free expression.

Professional athletes may well weather this latest storm. Still, to be free, black and disruptive today promises to remain a precarious status that has drawn the citizenship of all Americans into question. We have been warned. ​