Mexico is the United States' closest partner on immigration. Could that soon end?

For years, Mexico has stopped and deported large numbers of US-bound migrants in transit through its territory. Some are calling for that cooperation to stop.

Few countries have as much to lose from a Trump presidency as Mexico.

For the last few decades, its leaders have done their best to link their country’s fortunes to those of the United States, in everything from trade to security. Now, that formula is being tested by President Trump’s disparagement of Mexican immigrants, promises to levy tariffs on imports, and demands of compensation for a border wall, which led Mexican President Enrique Pena Nieto to cancel a meeting planned for this week.

Such moves threaten to derail talk of common interest, as pressure to retaliate builds on Mr. Pena Nieto.

But despite Mr. Trump's rhetoric, Mexico has fought to slow the flood of immigration from Central and South America to the US border. Trump's hostility to Mexican interests on trade might shift the locus of enforcement efforts away from Mexico, where it remains largely under the radar of the American public, and back to the US southern border.

“The US has been putting pressure for years [on Mexico] to put up obstacles to the transmigration from Central America,” says Jorge Bustamante, a sociologist at University of Notre Dame and emeritus professor of El Colegio de la Frontera Norte in Tijuana, Mexico.

US-funded initiatives for Mexican areas that border Guatemala and Belize have supported security cordons, naval bases, training for troops, surveillance equipment, and mobile kiosks that capture migrants’ biometric data – especially in the wake of the Central American women and children pouring into the US’s southern edge in 2014. Some $75 million of State Department funding has been allotted for coming years, according to a Congressional Research Service report released this month.

The funding has meant deportations, and lots of them. In 2015, Mexico deported 118,000 Central Americans, according to government data, and last year, the total number of deportees was more than 187,000.

“It’s not because it’s in Mexico’s interest,” Dr. Bustamante tells The Christian Science Monitor. “Mexico doesn’t have any qualms about immigration from Central America.”

Without Mexico's enforcement, many of those migrants might end up in US detention centers. Among other features of the executive orders on immigration Trump signed this week is a requirement that border agents detain everyone apprehended while crossing illegally – including, for instance, Central American children seeking asylum.

Of course, not everyone deported from Mexico in the past two years was en route to the United States. But the changing profile of immigration at the southern US border suggests that Mexico has been doing much of the United States’ enforcement work. In December, the Pew Research Center reported that in fiscal year 2016, more Central Americans than Mexicans were apprehended at the southern US border.

Now, some in Mexico are asking whether their government shouldn't "open the tap" to US-bound migration, notes Duncan Wood, the director of the Mexico Institute at the Wilson Center.

That could pose problems for public security in Mexico, he tells the Monitor, including more human rights abuses, and potentially the proliferation of camps along the US border.

"I wouldn't imagine there would be a directive from the Mexican government saying, 'Just let them all pass,' " he says. But they could scale back patrols in the southern states to "highlight the impact."

And Mexican immigration forces haven't always handled migrants gently in recent years. Officers with Mexico’s migration institute, which is in charge of immigration enforcement, have been dismissed for handing over migrants to the drug cartels that prey on them. Activists accuse officers of racial profiling and constitutional violations. And in the past four years, crimes against migrants in frontier states have shot up as much as 300 percent, reported Mexican media this month.

But as Trump outrages Mexico’s public and political class alike with early steps toward his pledges to deport millions, wall off the border, and institute big tariffs on US-bound goods, some are calling for the government to retaliate by ditching its cooperation on everything linked to US national security. 

Jorge Castaneda, a former foreign affairs secretary and professor at New York University, has suggested Mexico refuse to accept deportees who don’t have documentation of Mexican citizenship. And more recently, in a column for Mexico’s El Financier, he advocated for a few specific gestures:

“Suspend communications with US security, police and military sectors in Mexico; limit cooperation with them; expel … some DEA or ICE agents in Mexico; revise anti-terrorist and third-country visa policies,” he wrote.

"There is an asymmetry of power," says Bustamante. "Mexico didn't start this crisis."