The U.S. Ambassador to Mexico, in the final days of the Trump administration, "hit the roof" after Mexico pushed through new legislation in late 2020 that blocked Mexican detention centers from taking back migrants with children, according to a new book on the migrant crisis. 

Todd Bensman, a national security fellow at the Center for Immigration Studies, reports in his new book "Overrun: How Joe Biden Unleashed the Greatest Border Crisis in U.S. History" on the diplomatic fallout of a law passed in November 2020, just days after President Biden was elected.

The law, "Various Articles of the Migration Law and the Law on Refugees are Reformed, Complementary Protection and Political Asylum in the Matter of Migrant Children" went into effect on Jan. 11 2021, and meant that Mexican states had the authority to refuse U.S. expulsions of migrant families under the Title 42 public health order. The most significant of those states was Tamaulipas, one of the main traffic routes for migrants.

"The collective effect of the ‘reforms in favor of migrant children and adolescents, asylum seekers and refugees’ was that thousands of migrant families found that they were not only freed from Mexican detention centers, but that, when they crossed the U.S. border, the Americans would have to keep them," Bensman summarizes. "A massive breach was thus opened into America through which an unremitting onslaught of migrant families would pour for years."

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Mexican President Andres Manuel Lopez Obrador

Mexican President Andrés Manuel López Obrador listens as he meets with President Joe Biden in the Oval Office of the White House in Washington, Tuesday, July 12, 2022.  ((AP Photo/Susan Walsh))

In FY 2021 there were more than 1.7 million migrant encounters -- a new record, which was then exceeded by more than 2.3 million in FY 2022. FY 2023 has so far been on track to exceed that. In recent months the Biden administration has secured more cooperation from Mexico on Title 42 expulsions, which are due to end on May 11. 

But the move, which was not loudly trumpeted in the Mexican media, and was lost in the furor over the transition and Jan. 6 riot in the U.S., caused a diplomatic controversy. President Trump’s ambassador to Mexico, Christopher Landau, told Bensman in an interview that he heard of the move in a routine morning meeting shortly before Trump left office.

Bensman reports that Landau felt angered and betrayed that he had never been informed by Mexico about the brewing move.

"I really hit the roof on this," Landau said, adding that migration was the most important issue on the bilateral agenda. "And no one told me it was on the agenda. I remember thinking this is really going to screw us on some of these issues, especially if it’s implemented with any teeth." 

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Had he known it was coming, he added, "I would have been on this like a fly on honey." 

The crisis at the border continues to be an ongoing issue for U.S.-Mexican relations, and for the Biden administration -- which has faced continued political pressure from Republicans and some Democrats on the issue.

This week the administration announced a new rule that would make migrants ineligible for asylum if they had crossed through another country without seeking asylum there and if they enter the U.S. illegally between the ports of entry.

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Separately it has expanded a humanitarian parole program for Venezuelans to include Haitians, Nicaraguans, and Cubans. That program, accompanied by an expansion of Title 42 removals to those nationalities, allows for 30,000 migrants to enter the U.S. each month if they meet certain conditions. Republican states are suing, saying the program is unlawful.