President Trump in Washington on May 17. (Saul Loeb/AFP/Getty Images)

Even by President Trump’s pyrotechnic standards, his announcement on Thursday that he will impose a sweeping 5 percent tariff on all Mexican goods coming into the United States unless Mexico stops the flow of illegal immigration is unprecedented. The threat is unjustifiably heavy-handed and will further erode cooperation in bilateral relations as the contentious debate over immigration spills into areas that had been successfully compartmentalized.

Above all, Trump’s threat illustrates his absolute disinterest in reaching a sensible understanding.

The government of President Andrés Manuel López Obrador has shown unparalleled compliance with the White House’s punitive demands. It has increased the number of agents at its southern border, agreed to hold asylum seekers and dramatically increased deportations of potential asylum seekers.

Late on Thursday, López Obrador answered Washington with a long letter that included a lecture on American history, a brief declaration of discrepancy with Trump’s methods and a mellifluous plea for productive and urgent dialogue. Good luck with that.

Trump’s latest salvo also illustrates the profound rift in the different approaches to solve the humanitarian crisis that first began in Central America’s “Northern Triangle” of Guatemala, Honduras and El Salvador.

Early last week in Mexico City, Alicia Bárcena, head of the United Nations Economic Commission for Latin America and the Caribbean, revealed an ambitious development project for Mexico’s southeast and the troubled Northern Triangle.

“Why do people choose to leave?” Bárcena asked. “The lack of a basic source of income and economic opportunity is one of the main reasons.” She went on to explain how inequality, violence and global warming have also fueled the emergency. Bárcena then suggested what she called an “innovative” solution to the problem: Rather than focus on punishing measures to deter immigration, the region should instead emphasize growth through cooperation. López Obrador, sitting a few feet away, nodded. “This plan is important because it goes to the heart of the matter,” López Obrador later added. “People emigrate out of necessity. There’s no other way but to cooperate in search of development.”

But López Obrador’s words belied his own government’s actions.

Contrary to Trump’s unfounded complaints, Mexico has actually implemented myriad other, more bruising ways to try to stem the flow of immigrants toward the United States. In a somewhat schizophrenic policy, it has simultaneously slashed funding for the agencies assigned to handle refugees within the country while executing some of the most punitive schemes put in place by the Trump administration. Not exactly development-oriented actions.

Still, López Obrador insists that the only long-term solution to the current immigration crisis lies in opening new areas of opportunity for the hundreds of thousands of Central Americans who decide to migrate. All three Northern Triangle countries seem to agree: Diplomats for Honduras, Guatemala and El Salvador surrounded López Obrador for Bárcena’s presentation in Mexico City.

The problem, of course, is the one country missing from this seemingly unanimous show of goodwill: the United States.

For six months now, López Obrador has tried to persuade the Trump administration to invest billions in Central America rather than just focus on enforcement. Just a few days after Bárcena’s impassioned announcement, López Obrador dispatched Foreign Minister Marcelo Ebrard to sell Trump’s team on regional development. Ebrard didn’t go far. While he did meet with acting Homeland Security Secretary Kevin K. McAleenan and Jared Kushner, he was snubbed by Secretary of State Mike Pompeo, who canceled a previously scheduled meeting with his Mexican counterpart. Ebrard flew back empty-handed.

Is Mexico being naive? Clearly. To acquiesce to an investment project for Central America would require a complete about-face in Trump’s hostility toward the region. Before Trump announced that he will suspend all aid to Guatemala, Honduras and El Salvador as punishment for their supposed inaction to prevent the migrant exodus, the United States had assigned slightly more than $180 million in funding for the three countries combined in 2019, less than 2 percent of the amount Mexico would like to see the United States provide the area through aid and investment in the coming years.

Getting Trump to invest seems like a long shot. Just how long? The White House isn’t exactly masking his invective.

Aside from the drastic imposition of tariffs, the Trump administration is also apparently considering limiting the ability of potential migrants to request asylum in the United States if they have traveled by land through Mexico, a radical change that could create an unmanageable bottleneck and humanitarian crisis of catastrophic proportions for Mexico’s unprepared and underfunded government agencies.

As if that weren’t enough, consider McAleenan’s visit to Central America this week. McAleenan did indeed carry with him a message of collaboration, but certainly not in the areas Ebrard and Bárcena might have hoped for.

On Wednesday, McAleenan met with the Guatemalan Ministry of Government to sign a formal memorandum of cooperation that focuses almost exclusively on enforcement. “Both countries have agreed to take concrete actions necessary to combat the scourge of human trafficking and smuggling, interdict illicit drug trafficking, and target illegal trade and financial flows,” the Department of Homeland Security explained in a statement. “This will include law enforcement training and collaboration to improve criminal investigations.”

The region’s long-term development merited only the vaguest of mentions. In theory, DHS said, the agreement will “improve the ability of both countries to identify and better understand” the root causes of immigration. That’s a long way from the kind of commitment needed to rebuild an impoverished, violent and drought-stricken region.

On Wednesday, I asked a spokesman for Mexico’s foreign ministry about the development plan’s outlook if the Trump administration ultimately declines to join. “Their support is important,” he told me. “But we don’t need the United States. This is our plan.”

This bravado is misguided. The United States is not just another actor in the current drama. Without it — or worse, with the Trump administration as rabid antagonist — a regional bet on Central America’s future will face impossible odds.

Read more:

León Krauze: Mexico wants to scrap the Mérida Initiative. That would be a terrible mistake.

León Krauze: Trump’s self-defeating plan to end aid to Central America

Max Boot: Stop demonizing Mexico. Start understanding it.

The Post’s View: Trump’s huffing and puffing at the border won’t make the migration problem disappear