The language tucked into Trump's order on building a border wall instructs the Department of Homeland Security to use an obscure federal law to send land-arriving migrants back to neighboring territory, even as they continue to await court proceedings in the United States.
The policy shift would mean that even if they are thought to have a credible claim to asylum, they may be forced to wait years in Mexico for a future court date in the US.
The way the law is structured, most of the affected individuals would likely be Central American migrants fleeing violence and drug cartels in their home countries by making the trek north through Mexico.
As non-Mexican citizens, Mexico would not be obligated to take these individuals in -- and the move comes at a time when relations between the two countries are already at a historically tense point.
Critics of Trump's approach to immigration say the policy is a solution in search of a problem and in fact causes more issues than it fixes both in terms of relations with Mexico and the impact on the individual lives involved.
Supporters of the move, who largely hail from the conservative school of thought on immigration policy, say it serves two purposes. First, it could press Mexico to do more to stop the flow of Central American and other migrants north through its territory as they seek a way into the US.
Second, they argue, it would begin to stop what they see as a persistent issue of immigrants seeking a life in the US presenting themselves at the border, claiming a right to asylum and then disappearing as they await final judgment on their case.
The provision in question is Section 7 of Trump's border security order last week. "The Secretary shall take appropriate action, consistent with the requirements of section 1232 of title 8, United States Code, to ensure that aliens described in section 235(b)(2)(C) of the INA (8 U.S.C. 1225(b)(2)(C)) are returned to the territory from which they came pending a formal removal proceeding."
A US government source familiar with the draft implementation documents for the order confirms that the intent is to use US law to send asylum seekers to Mexico, or potentially Canada if that's where they arrive from, to await their formal hearing.
A Trump administration official would not say how often the White House intends to use the maneuver. "'Appropriate action' does nothing more than preserve our options with respect to many different policies and procedures," the official said.
But the policy could impact tens of thousands of people per year, according to government statistics.
The Trump administration has already kicked off under tense relations with Mexico -- from threatening tariffs on products manufactured there, to insisting the government will reimburse the US for a wall along its border.
Adding on an attempt to force Mexico to accept tens of thousands of non-Mexican families to wait for years on a decision in the US could further damage the already tense relationship -- at a time when the US still needs Mexico's cooperation on a host of issues including border security and drug trafficking.
"I can't see the Mexicans accepting that," said Jeffery Davidow, a senior counselor at the Cohen Group and former career foreign service officer and ambassador to Mexico from 1998 to 2002. "Asylum seekers should be the concern and responsibility of the country in which they're seeking asylum, and if they make it to the United States, I don't see the Mexican government taking on that responsibility."
How it works
When migrants present themselves at the border with the US without valid entry documents, they are usually placed in expedited removal proceedings. But if they say they are seeking asylum, they are given what's called a "credible fear" interview. If after a series of questions an immigration officer determines they may have a claim to asylum, they are given a court date, often years in the future.
In fiscal year 2016, according to US Citizenship and Immigration Services numbers, 92,990 decisions were made nationwide on credible fear, and in 73,081 cases, fear was established -- almost 80% of the cases.
Immigration hawks argue that too many of these individuals never show up for court, instead living illegally in the US.
Rather than keeping these people in detention centers in the US for years, Section 7 would allow the US to send them back to the contiguous territory from which they entered.
"Mexico has not really been forced to deal with this stream of migration through their country as long as the US was accepting them, so this change in policy is going to force them to have to rethink their policy on these people," said Jessica Vaughan of the conservative immigration policy Center for Immigration Studies.
The US government official who works on border security, who spoke on condition of anonymity to not get ahead of the administration, said the policy change would likely go over well enough on Capitol Hill, even among more moderate Republicans.
"This helps put Mexico on the hook for holding and detaining these people until their asylum claims are adjudicated, and what that means is it's a great incentive for the Mexicans to actually cooperate with us to secure our Southern border, so Mexico has skin in the game," the official said. "Right now Mexico can take these people and send them north and it's not Mexico's problem. This makes it Mexico's problem."
The official cited Customs and Border Patrol numbers of more than 16,000 family units and more than 7,000 unaccompanied children apprehended at the southwest border in December alone, part of a steady uptick in the last quarter of 2016.
'Flashpoint of provocation'
It's not clear, however, why Mexico would agree to the change in policy. Mexicans can already be sent back to Mexico, but the majority of asylum seekers at the border come from elsewhere, often fleeing violence and cartels in Central America. If Mexico were to decline to accept migrants, the situation could escalate if the US were to try to force them to.
The Mexican embassy in Washington and President's office in Mexico City did not respond to a request for comment on the policy shift.
Karen Musalo, a professor at University of California Hastings law school specializing in refugee studies, says that Mexico has already been doing "dirty work for the US" in terms of intercepting migrants making their way north and working to shut down drug smugglers.
"This could be another flashpoint of provocation," Musalo said. "Treating these people like lawbreakers, it's not the appropriate narrative, because people fleeing persecution, they're entitled to seek protection. And if they don't qualify for protection, then they're likely to be returned, but let's not start from the assumption that these people are criminals."