For decades, thousands of people left El Salvador and crossed the border, taking up residence in a new country and gradually putting down roots.

The Salvadorans occupied areas that natives ignored or disdained, and they worked in­cred­ibly hard. Over time, however, unease over their presence grew, until the host government gave them a deadline by which to establish lawful residency or face deportation.

This is not a news update from the United States, where the Trump administration has told some 200,000 Salvadorans that the "temporary" haven they were granted in 2001, after an earthquake devastated their country, will expire in 18 months.

Rather, it is a summary of events in Honduras nearly 50 years ago, when that country's authorities informed tens of thousands of Salvadoran squatters that they would have to leave the land they had been farming so it could be distributed to Hondurans.

DHS announced May 4 that it will end protected immigration status for 50,000 Hondurans living in the U.S. since 1999. This is what you need to know about TPS. (Melissa Macaya, Claritza Jimenez/The Washington Post)

The crisis, aggravated by fan violence at World Cup qualifying matches between the two countries' teams, led to the brief, misnamed Soccer War in July 1969 — followed by an exodus of Salvadorans from Honduras.

This forgotten history has contemporary lessons, which we should try to understand lest President Trump's policy prove not merely morally questionable but also counterproductive.

El Salvador is the most densely populated Spanish-speaking country on the planet; yet a small elite historically controlled its best farmlands.

The struggle for existence there is intense, sometimes violent. And so generations of Salvadorans have left in search of land and work — and tranquility. Neighboring Honduras was once a crucial demographic escape valve. The 1969 war closed it, and disrupted the Central American common market, destabilizing El Salvador politically. There was a savage 1979-1992 civil war between U.S.-supported governments and Marxist guerrillas.

That conflict drove hundreds of thousands to the United States, establishing a migratory pattern that continues to this day. The 2.1 million Salvadoran-origin people now constitute the third-largest Hispanic group in the United States, after those of Mexican and Puerto Rican origin, according to the Pew Research Center.

Salvadoran labor helped build the shiny new downtown of Washington, D.C., one of several cities — including Houston and Los Angeles — that would barely be recognizable anymore without a Salvadoran community.

The Soccer War "functioned as a social and political bomb for El Salvador," Joaquín Villalobos, a Salvadoran guerrilla chieftain- ­turned-political-analyst, wrote in a recent issue of the Mexican magazine Nexos. The war catalyzed "a quarter-century of political convulsion and violence," which spread to the rest of Central America, Honduras included.

All the while, U.S. policy oscillated: between obsession and neglect; support for reform and support for repression. The latest turn in the cycle came under President Barack Obama, who offered both development aid and police funds to help stem a wave of unaccompanied youth migration during his presidency — with mixed results.

Villalobos may overstate his case. Certainly, in faulting the United States, he implicitly plays down his own historical agency and that of his fellow Salvadorans.

Still, he is correct to focus on the deeper causes of migration, and the United States' chronic failure positively to affect them. At the very least, history provides cause for concern that, by ending "temporary protected status" next year for nearly one-tenth of all Salvadoran-origin people here, Trump might ultimately destabilize Central America further.

Undoubtedly, the Salvadoran migration has had costs as well as benefits for the United States: Like certain previous migrations from Europe and Asia, the Salvadoran wave brought in a violent organized-crime group, MS-13.

The vast majority of Salvadorans work hard and live peacefully, however, as did the vast majority of the Salvadorans in Honduras whom that country endeavored to expel, with such disastrous results, 50 years ago.

Deporting 200,000 now would be true to the letter of a U.S. humanitarian law that was never intended to provide permanent residency.

At the same time, it would deprive the Salvadoran economy of millions of dollars in cash remittances, while requiring it to house and employ a large number of returnees.

Of course, that's on the implausible assumption that most affected Salvadorans wouldn't try to stay, thus swelling the very undocumented population Trump is supposedly bent on shrinking.

MS-13 itself metastasized in El Salvador as the unintended consequence of a (defensible) American effort, begun under the Clinton administration, to deport members convicted of crimes in the United States. The gang began in L.A.'s Salvadoran community; once back in El Salvador, its members took advantage of corrupt, weak law enforcement to expand and, eventually, reach back into the United States.

Of all the United States' international relationships, surely the most underrated — in terms of tangible impact on people's everyday lives, both here and abroad — is the one with El Salvador. Any policy that fails to take that into account is doomed to fail.

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