The Mexican border police officer hung from the back of the truck rumbling through the streets of Tapachula, near the border with Guatemala, one hand on the overhead roll bar and the other on his AR-15 rifle. Made in the USA, he said. So was his bulletproof vest. And much of his training.

His unit is meant to help migrants, but his gear underscored the close relationship – and direct assistance – that Mexican security forces have with their counterparts in the United States, a relationship that has matured in recent years to reach unprecedented cooperation combating drugs, transnational crime, and migration.

That’s the reality that Homeland Security Secretary John Kelly and Secretary of State Rex Tillerson will confront in Mexico this week, as they make the hard sell to Mexican President Enrique Peña Nieto and other officials on President Donald Trump’s tough new immigration directives.

Released publicly on Tuesday, the new orders rescind most all of President Barack Obama’s orders and guidance on immigration, staking out a much tougher line on undocumented migrants in the United States. They make it easier for U.S. officials to deport huge numbers of immigrants simply for having entered illegally. They also expand “expedited removal” without a hearing for those caught within two years of entering and anywhere the United States, a clear break with the Obama administration, which only used the practice for those apprehended within two weeks of entering and within 100 miles of the U.S. border. The directives would even deport foreign nationals to countries other than their own. (Read the orders and .)

“There’s a misperception that because the U.S. is much bigger and in many ways more powerful, that Mexico lacks leverage,” Chris Wilson, the deputy director of the Mexico Institute at the Woodrow Wilson International Center for Scholars, told Foreign Policy. “Right now, Mexico is deporting more Central Americans than the U.S., so many would say Mexico is doing some of the United States’ dirty work … all of that – and Mexico has basically stated as much – is on the line if the cooperative framework of the relationship is lost.”

On Wednesday ahead of meetings with Kelly and Tillerson, Foreign Minister Luis Videgaray said Mexico will not accept ” unilateral ” U.S. immigration proposals, the main topic on the agenda for discussions into Thursday.

But the White House hasn’t helped allay fears. While saying immigrants who pose a threat to public safety are the priority for deportations, Press Secretary Sean Spicer noted Tuesday that “everybody who is here illegally is subject to removal at any time.”

Trump has seriously strained U.S.-Mexico relations since before he took office, threatening trade wars, working on a border wall, and even talking of sending the U.S. military across the border to confront ” .” (Peña Nieto cancelled a trip to Washington last month.) On Tuesday, Mexico’s incoming ambassador to the United States said Trump’s treatment of Mexico is ” bad hombres unacceptable.”

It’s left to Kelly to do the difficult work of translating the new president’s bluster into policy guidance for the front-line border patrol agents who have to implement it – and the alarmed heads of state they need to make it work.

That’s especially important because the biggest immigration headache for the United States doesn’t originate in Mexico – but requires Mexico’s help to tackle.

Illegal crossings into the United States from Mexico have sunk to their lowest levels in four decades, and among Mexican immigrants, the flow has in fact since 2009, at the tail end of the Great Recession. Rather, it’s the violence-ridden countries of the “Golden Triangle” – Guatemala, Honduras, and El Salvador – that have been the primary drivers of migration through Mexico to the United States in recent years, as seen in spikes of unaccompanied minors at the border. And it’s not just Central Americans. Global migration – from Haiti and Cuba to Cameroon and Somalia – has pushed more people to use the dangerous land route to the United States.

Under a grab bag of different programs, Washington has provided billions of dollars in assistance to Mexico and those Central American countries to stop their citizens from reaching the United States. Mexico has responded, dramatically stepping up its detentions and deportations, with removals from 2013 to 2016, primarily of Central Americans. And more people are deciding to stay in Mexico and apply for refugee status there, rather than trying to cross into the United States. Trump’s latest orders call for auditing “all sources of direct or indirect” federal aid to Mexico, hinting at cuts.

But the Trump administration’s new rules on immigration enforcement could sour that relationship and paradoxically drive more immigration northward. The new rules could give Mexico more leverage to push back against Washington, especially if the recently stepped up enforcement at Mexico’s southern border were scaled back or if U.S. funding dried up.

Mexico, though, would have to agree to that and has indicated it’s unlikely to be willing to hold thousands of immigrants as they await their proceedings with U.S. officials. Additionally, a 2001 Supreme Court ruling found that if a country of origin doesn’t agree to take a migrant back, he or she must be released from detention.

“If the United States starts sending non-Mexicans back to Mexico, Mexico doesn’t have to receive them,” Wilson said. “Just like the U.S. gets to determine who is admitted, so does Mexico.”

Kelly at least, though dealt a tough hand as an emissary for Trump policies that have sparked mass protests in Mexico, has substantial experience with security counterparts in the region thanks to his former leadership of U.S. Southern Command, which stretches from the Mexican border to the southern cone. He has spent time on both sides of Mexico’s northern and southern borders and worked extensively with the governments of Guatemala, Honduras, and El Salvador.

That experience helped inform a more nuanced view of immigration than Trump has often espoused. But it remains to be seen how much of a tempering influence Kelly has on the president.

Still, in his confirmation hearing before the Senate last month, Kelly praised the progress and importance of regional security forces to stanching the northward flow of migrants. In order to really stop drugs and people from reaching the U.S. border, he said the oft-touted “wall” should be built much farther south.

This piece was updated at 12:45 p.m. Wednesday to reflect Foreign Minister Videgaray’s remarks. Reporting for this article was also supported by the International Women’s Media Foundation as part of the Adelante Latin America Reporting Initiative.

Photo credit: Justin Sullivan/Staff