However, the military cannot act as a police force and make arrests or use force.
Texas Gov. Greg Abbott said Monday that a small number of Texas National Guard members would remain on the U.S.-Mexico border, where they have been for the past three years under his authority, conducting surveillance. They will begin taking federal orders this month, he added.
About 100 service members will remain in the border region, The Associated Press reported, citing Lt. Col. Travis Walters, a spokesman with the Texas Air National Guard.
5. How is Mexico reacting?
The Mexican ambassador to the U.S., Gernimo Gutirrez, said his government has asked for clarification from the Trump administration.
"It's certainly not something that the Mexican government welcomes," he told CNN.
In an interview with NPR on Tuesday, Gutirrez said, "We're trying to find the best ways to manage migration flows in a way that it's respectful both to the laws of the United States and Mexico."
He also said most migrants reaching the Southern U.S. border are "no longer Mexicans; they are Central Americans."
The immigration issue is a hot-button topic in Mexico's presidential campaign.
The leading candidate, Andrs Manuel Lpez Obrador, said this week that "Mexico and its people will not be the piata of any foreign government."
As the crowd jeered at the reference to Trump, Lpez Obrador added: "It's not with walls or use of force that you resolve social problems."