Air Force Gen. Terrence J. O’Shaughnessy, the chief of U.S. Northern Command, told reporters Monday that the deployments, dubbed “Operation Faithful Patriot,” already are underway. He said the military, working alongside U.S. Customs and Border Protection (CBP), will focus first on “hardening” the border in Texas, followed by Arizona and California.

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The mobilization will include three combat engineer battalions, members of the U.S. Army Corps of Engineers and troops who specialize in aviation, medical treatment and logistics, O’Shaughnessy said. Black Hawk helicopters sent to the area will have night-vision capabilities and sensors, carrying troops trained in the kind of aerial combat missions used by the military in active war zones.

“We’ll be able to spot and identify groups and rapidly deploy CBP personnel where they are needed,” O’Shaughnessy said.

The activation of such a large contingent of active-duty forces at the border — as opposed to National Guard troops — has no modern precedent and appeared to be the largest of its kind in a century during peacetime.

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Kevin K. McAleenan, the top U.S. border security official, said the decision to send troops was not motivated by electoral politics but rather was a law enforcement necessity, as U.S. agents prepare for the possibility that crowds of migrants will amass and turn unruly.

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McAleenan, the CBP commissioner, said his agency was tracking a caravan of about 3,500 people moving north through southern Mexico. A second group of about 3,000 that clashed with Mexican police over the weekend lags behind them.

“We are preparing for the contingency of large groups of arriving persons in the next several weeks,” he said. “We will not allow a large group to enter the United States in an unsafe and unlawful manner.”

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In private, Homeland Security officials acknowledge they were alarmed by scenes of migrants breaking through a gate at Guatemala’s border with Mexico last week. The concentration of so many people at the U.S. border, the officials said, would amount to a potentially volatile and unprecedented crowd-control challenge for U.S. border agents and customs officers.

“What is new and challenging about this caravan phenomenon is the formation of multiple large groups that present unique safety and border security threats,” McAleenan said.

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Last month, U.S. agents arrested a record number of migrant family members along the border, and more than half of those taken into custody now are parents with children or minors traveling along, McAleenan said.

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“We’re already facing a border security and humanitarian crisis,” he said, adding that an average of 1,900 people have been crossing the border without authorization each day in recent weeks.

Immigrant advocacy groups and the American Civil Liberties Union blasted the move to send military forces and said migrants are exercising their rights under international and federal laws to seek asylum in the United States. Many are walking toward the United States with their families and say they are escaping violence and grinding poverty back home.

“These migrants need water, diapers and basic necessities — not an army division,” Shaw Drake, policy counsel for the ACLU’s Border Rights Center in El Paso, said in a statement. “Sending active military forces to our southern border is not only a huge waste of taxpayer money but an unnecessary course of action that will further terrorize and militarize our border communities.”

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The active-duty deployment comes in addition to Operation Guardian Support, a National Guard mission launched last spring that involves just short of 2,100 troops. O’Shaughnessy, asked why the National Guard was not selected for the new mission, said the military is bringing “additional capability” with the active-duty troops. However, nearly all of the kinds of troops sought for Faithful Patriot exist in the Guard.

Initial deployments for the operation include soldiers from Fort Campbell and Fort Knox in Kentucky, Fort Hood in Texas and Fort Riley in Kansas, according to a Pentagon document obtained by The Washington Post. Those deployments, which primarily include military police and engineers, began Monday and will continue throughout the week, according to the document.

A Pentagon activation message for the operation distributed internally Monday states that Homeland Security has requested that the Defense Department provide support to the CBP “through the arrival and detention of the migrant caravan currently traveling to the U.S. southern border no later than 30 October.”

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The support will enhance the CBP’s ability to “impede or deny illegal crossings, maintain situational awareness and apply the appropriate law enforcement response as it contributes to the overall border security mission of CBP,” the message said.

The Trump administration's selection of active-duty service members for the new operation also will eliminate complications with governors who do not want their National Guardsmen involved in the operation. In June, several governors called home Guardsmen from the border amid outrage about the Trump administration's separation of migrant children from their parents at the border.

O’Shaughnessy said that the personnel on the border will continue to follow the Posse Comitatus Act, which prevents U.S. troops from taking a direct role in law enforcement missions in most circumstances.

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O’Shaughnessy said the Pentagon also will deploy military police units and cargo aircraft, including three C-130s and one C-17. Combined command posts will be established to integrate U.S. military and CBP efforts.

The Pentagon already has sent 22 miles of concertina wire to the 1,954-mile-long border, he said, and has enough additional wire to cover 150 miles.

The deployments thrust the military further into a political fight in which the president increasingly has sought to cast the migrants as a national security threat in the days leading up to the Nov. 6 midterm elections. Earlier Monday, Trump tweeted accusations about the caravan without citing any evidence.

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“Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border,” Trump said. “Please go back, you will not be admitted into the United States unless you go through the legal process. This is an invasion of our Country and our Military is waiting for you!”

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The White House has shown confidence that Trump’s hard-line enforcement message will continue to drive his conservative base to the polls and even draw some crossover appeal among more-moderate voters.

White House press secretary Sarah Sanders said the administration is considering several administrative actions on the southern border, although she declined to publicly describe the options. Trump will do what “he deems necessary” on immigration, Sanders said.

The White House has put significant pressure on the government of Mexico to block the caravan’s advance. The group has diminished from a peak of nearly 7,000 migrants, as some footsore travelers and parents with children have dropped out or fallen behind. At least 1,000 caravan members have applied for asylum in Mexico, authorities say.

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Mexican President Enrique Peña Nieto on Friday offered temporary work permits, medical care and other benefits to migrants if they agree to register with authorities and remain in the Mexican states of Chiapas and Oaxaca, far from the U.S. border. But the core group of mostly Honduran migrants has rejected his entreaty and continued heading north toward the U.S. border.

The caravan remains at least 900 miles from U.S. territory, so its arrival is not imminent.

In an attempt to limit the caravan’s size, Mexican police clashed Sunday with a smaller, separate group of Central Americans attempting to enter from Guatemala and catch up to the main group. At least one man was killed as police fired rubber bullets and tear gas.