Trump Jr.'s tweet came in response to Mexican Secretary of Defense Luis Sandoval saying in Cancun on Monday: "In the northern part of the country, we have deployed a total of nearly 15,000 troops composed of National Guard elements and military units".
Defense Minister Luis Cresencio Sandoval said the personnel is a mix of army units and the country's national guard, and he acknowledged they are detaining migrants preparing to cross into the U.S.
"Given that (undocumented) migration is not a crime but rather an administrative violation, we simply detain them and turn them over to the authorities" at the National Migration Institute.
Mexico says it has sent almost 15,000 troops to the USA border. Mexico, however, has also called on the United Nations and the United States to help with the monumental task. "That's why we're providing support, it's a strategy being pursued on both borders".
Lopez Obrador, an anti-establishment leftist who came to office in December vowing to protect migrants' rights, has been pushed into a more hardline stance by Trump, who threatened last month to impose tariffs on all Mexican goods if the government did not do more to slow migration.
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The Mexican government is working with the governments of Honduras, El Salvador and Guatemala to limit the flow of migrants from those countries, while the US Department of Homeland Security reportedly sent dozens of agents to Guatemala to "disrupt and interdict human smuggling operations" along the country's border with Mexico.
Former Mexican national security official Gustavo Mohar said Mexico's security forces had not been used this way before, describing the development as "sad". USA border security officials say an unknown number of these units use fraudulent birth certificates to fake family ties. As a result, those who claim asylum as families are released into the United States shortly after apprehension.
President Andres Manuel Lopez Obrador unveiled his plan at a news conference in Mexico City on Monday.
Trump had said earlier that Democrats have forced him to take action against Mexico because they had not been willing to fix loopholes in US immigration laws that are fueling the immigration crisis.
However, worldwide law protects the right of undocumented migrants to cross global borders to request asylum.