Shaw Drake, policy counsel at its rights centre in El Paso, Texas, said: "President Trump has chosen just before midterm elections to force the military into furthering his anti-immigrant agenda of fear and division".
"According to what they say, we are not going to be very welcome at the border", Honduran migrant Levin Guillen said when asked about Trump.
He is expected to hold 11 rallies in the days ahead of the November 6 midterms, which Washington pundits are characterizing as a referendum on his presidency. Some of the troops will be armed.
Because the Posse Comitatus Act prohibits the USA military from performing law enforcement activities within the United States, the troops will be in support roles only.
The new group, whose members called themselves a second caravan, gathered on the global bridge leading from Tecun Uman to Mexico.
The caravan of migrants clash with the Guatemalan Police, in Tecun Uman, Guatemala. Others joined it in southern Mexico. That group appeared to begin as a collection of about 160 who chose to band together in Honduras for protection against the gangs who prey on migrants traveling alone and snowballed as the group moved north.
Despite carrying weapons, the MP forces would not be authorized to conduct law enforcement operations, an activity that would be barred under the 1878 Posse Comitatus Act.
Mexico, meanwhile, has also been tightening security on its southern border, where migrants often use rafts to cross the Suchiate river. Under worldwide law, these are considered refugees.
The Defense Department says it's sending 5,200 active duty troops to "harden" the southern border against a migrant caravan making its way through Mexico. He said the caravans was "different" than those seen by the United States in the past, adding that they were better organized. "We want to work with Mexico to manage that flow".More news: Trump slams protests as 'staged' and 'fake news' after Pittsburgh visit
"The Mexican government rejects the violence that occurred today on the border with Guatemala, and reiterates that the only way to enter Mexico is in compliance with immigration laws", Mexican interior minister Alfonso Navarrete said during a press conference on Sunday.
The military operation drew quick criticism. "We will continue to adjust the number and inform you of those", O'Shaughnessy told reporters. That law bans the use of American military personnel for civilian law enforcement efforts on USA soil, outside military installations.
The announcement comes just days after President Trump declared the migrant caravan making its way to the US a "National Emergency". "It's going to be the election of common sense", Trump said at a rally in IL on Saturday night.
Trump said anyone wanting to get into the United States must do so through the legal process.
On Monday he warned that the military would be waiting.
It's possible there are criminals mixed in, but Trump has not substantiated his claim that members of the MS-13 gang, in particular, are among them.
They will bring border personnel "exactly where they need to be, regardless of the conditions", O'Shaughnessy said.
The projected USA deployment is already roughly the same size as the US military contingent in Iraq. "But please know that is in addition to the 2,092 that are already employed from our national guard troops".