But the mission could jeopardize Mattis’s effort to shield the military from the political divisions gripping the country as questions persist about the size and timing of the deployment and the use of active-duty troops instead of the National Guard.

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In his nearly two years as defense secretary, Mattis has sought to protect the military from the sort of politicization that has rattled the FBI and the CIA since President Trump took office.

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The Pentagon says the large-scale deployment is necessary to harden points of entry and deter a caravan of an estimated 3,000 to 7,000 Central American migrants that is about 900 miles south of the border and making its way northward.

Representatives for Mattis did not respond to requests for comment, but other officials have rejected the notion that the scale and timing of the deployment has political motives and say a high-profile, rapid rollout could serve as a deterrent.

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“We’ve got to be prepared for the potential arrival of a very large group,” Kevin McAleenan, commissioner of U.S. Customs and Border Protection (CBP), said at a news conference Monday. “Because of the size, we want to be able to handle it effectively and safely.”

Critics, meanwhile, say the decision to deploy roughly the same number of U.S. troops serving in Iraq to the border in the days before the midterm elections far exceeds the threat at hand. The caravan is three to four weeks away from arriving, the critics note, and the deployment helps Trump focus attention on an issue ahead of the Nov. 6 vote that could energize his base.

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Internal Pentagon documents obtained by The Washington Post indicate that the military specifically wants to highlight the deployment in part to “deter or dissuade the caravan” from continuing toward the United States.

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“This is a craven political stunt by President Trump ahead of the U.S. midterm elections, and a cynical capitulation by a secretary of defense who has prided himself on improving the readiness, focus, and lethality of the U.S. armed forces and who was once known as a no-nonsense warfighter,” Kelly Magsamen, a former top Pentagon official in the Obama administration and now a vice president at the liberal Center for American Progress, wrote in an op-ed for Defense One.

Mattis has said little in public about the deployment, other than to confirm during a news conference Sunday in the Czech Republic that the troops would conform with all U.S. laws while serving on the border. Because U.S. law prohibits the military from domestic policing duties, the troops will be limited to helping CBP officers do their jobs.

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Earlier on Sunday, Mattis described the mission as “defense support for civilian authorities” and said whatever Border Patrol needed would get there, but he didn’t go into detail about the rationale or the threat.

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He didn’t discuss the deployment at an appearance in Washington at the U.S. Institute of Peace on Tuesday.

Gen. Terrence O’Shaughnessy, commander of U.S. Northern Command, which is overseeing the operation, said Tuesday at the Pentagon that additional forces could follow the 5,239 troops being sent to the border. He said 2,092 National Guard forces are already broadly assigned to the mission there, operating under an April order from Trump. Texas will receive the active-duty forces first, O’Shaughnessy said, and will have about 1,800 in place within days.

The Air Force general said CBP personnel will be the primary officials dealing with migrants but that U.S. service members deployed to support them would be properly trained in case of any incidental interaction.

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Some of the active-duty troops heading to the border will deploy with weapons, O’Shaughnessy said a day earlier. According to the internal Pentagon documents reviewed by The Post, active-duty personnel can use force to protect themselves or others.

The Marine Corps will deploy a special-purpose task force headquarters as part of the U.S. military’s mission, while the Army’s 82nd Airborne Division will deploy helicopters, according to the documents. The Army also will deploy several military police units. Troops will deploy with a mixture of lethal and nonlethal weapons and are authorized to use deadly force in defense of “all persons, foreign or domestic, who are faced with imminent threat of death or serious bodily harm, and where lesser means have failed or cannot be reasonably employed.”

The military operation, dubbed Faithful Patriot, is designed to help the CBP “impede or deny illegal crossings, maintain situational awareness and apply the appropriate law enforcement response,” according to the documents.

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While the mission is due to conclude on Dec. 15, one document suggested the active-duty presence could be curtailed “once the migrant caravan has reached a capacity that CBP can process” on its own.

Among the activities the troops are expected to tackle are constructing barriers and fencing, using helicopters and fixed-wing aircraft to move CBP personnel, and dispensing medical care. The government has also asked the Pentagon to provide command and control facilities, temporary housing for CBP personnel, and riot gear for 500 CBP employees. Military medical teams will be primarily tasked with treating government personnel and are not expected to take part in “medical screening” of migrants.

“No DOD civilian or military personnel are involved in the transportation, lodging, care or administration of migrants,” one document said.

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The troops are operating in a support role for CBP because a law known as the Posse Comitatus Act generally prohibits the U.S. military from direct participation in law enforcement.

Jason Dempsey, a former Army officer who serves as an adjunct senior fellow at the Center for a New American Security, said the deployment raised questions about how Mattis was balancing his goal of making the U.S. military a more lethal, focused organization with pressure to get involved in the president’s political priorities.

The balancing act has been a strain for Mattis since his first week at the Pentagon, when Trump signed his controversial travel ban in the Pentagon’s Hall of Heroes alongside Mattis, who like other military leaders, received little warning about Trump’s plans.

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The use of active-duty forces enables a faster mobilization and is likely to be cheaper than calling up members of the National Guard because additional pay and benefits are required when the Pentagon activates forces from the Guard and reserves.

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A two-year deployment of up to 6,000 guard troops to the border during President George W. Bush’s administration cost about $1.2 billion. The Obama administration spent about $110 million over one year to deploy up to 1,200 to the border, according to the Government Accountability Office. O’Shaughnessy said Tuesday that he couldn’t give a cost figure for the current deployment.

But the decision to use active-duty forces also threatens to pull troops away from training activities and preparation for overseas deployments, and it could further stretch a force that military leaders for years have complained is beleaguered by excessive deployments.

The Pentagon is aware of the risks a deployment within the United States poses to the military’s public reputation if something goes awry. Mattis, a retired Marine Corps general, was on active duty in 1997 when a U.S. Marine, deployed to the border on an anti-drug mission, shot and killed an American teenager tending to his herd of goats.

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Charles J. Dunlap Jr., a retired Air Force major general and law professor at Duke University, said the use of the regular armed forces within the United States for politically charged goals is seldom desired by the military itself.

“In an era when the vitality of our armed forces depends upon the best and brightest volunteering, it’s hard to overstate how important it is for the military to maintain the strong public support and admiration it currently enjoys,” Dunlap said in an email. “Involvement in controversial domestic matters inevitably risks alienating at least part of the citizenry.”

Dunlap said there are times when regular troops are the best option on short notice, and they can offer professionalism at tumultuous times, citing President Dwight D. Eisenhower’s decision to order elements of the 101st Airborne Division to help desegregate Little Rock schools in 1957.

“If we are faced with a concentration of thousands of people unlawfully crossing the border, having the military provide the rear-echelon support and logistics so as to free law enforcement professionals to keep public order in the front line, so to speak, may be the best or even only near-term option,” Dunlap said.