Every president since Reagan has deployed troops to the border

"We're going to do things militarily. Until we can have a wall and proper security, we're going to be guarding our border with the military," President Donald Trump said on Tuesday.

"We really haven't done that before, or certainly not very much before," he added.

About 24 hours after Trump said the U.S. hasn't often used the military to guard the border, Homeland Security Secretary Kirstjen Nielsen talked about such past military operations as inspiration.

"Today, what we've discussed are support activities that are very similar to Jump Start," Nielsen said.

SEE MORE: Who Would Trump's Wall Allegedly Prevent From Crossing The Border?

"They include everything from aerial surveillance, which is as you know was part of Operation Phalanx," she added.

It turns out, every president since Ronald Reagan has authorized temporary military missions at the border. But those missions focused on surveillance and logistical support, not on arrests and use of force. That's because an 1878 law limits the military's power to conduct police action within the U.S.

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President Trump visits border wall prototypes amid protests
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President Trump visits border wall prototypes amid protests
US President Donald Trump is shown border wall prototypes in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
People hold signs during a protest while standing in front of the current border fence and near the prototypes of U.S. President Donald Trump's border wall, in Tijuana, Mexico March 13, 2018. The sign on the right reads "Trump, walls can be jumped over". REUTERS/Edgard Garrido
US President Donald Trump inspects border wall prototypes in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
People attach a sign reading "Trump, stop the mass deportations" to the current border fence and near the prototypes of U.S. President Donald Trump's border wall, during a protest in Tijuana, Mexico March 13, 2018. REUTERS/Edgard Garrido
US President Donald Trump's motorcade arrives at the border fence in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
People hold signs reading "No to the wall" and "Trump, put your wall, but in your territory, not in ours", during a protest near the prototypes of U.S. President Donald Trump's border wall, seen behind the current border fence, in Tijuana, Mexico March 13, 2018. REUTERS/Edgard Garrido
US President Donald Trump is shown border wall prototypes with White House Chief of Staff John Kelly (L) in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
A man holds a sign reading "Trump, put your wall, but in your territory, not in ours", during a protest near the prototypes of U.S. President Donald Trump's border wall, seen behind the current border fence, in Tijuana, Mexico March 13, 2018. REUTERS/Edgard Garrido
US President Donald Trump arrives to inspect border wall prototypes in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
People hold signs reading "No to the wall, Trump," and "Trump, we are not enemies of the USA" during a protest near the prototypes of U.S. President Donald Trump's border wall, seen behind the current border fence, in Tijuana, Mexico March 13, 2018. REUTERS/Edgard Garrido
US President Donald Trump speaks during an inspection of border wall prototypes in San Diego, California on March 13, 2018. Donald Trump -- making his first trip to California as president -- warned there would be 'bedlam' without the controversial wall he wants to build on the border with Mexico, as he inspected several prototype barriers. The trip to the 'Golden State' -- the most populous in the country and a Democratic stronghold -- was largely upstaged by his own announcement that he had sacked Secretary of State Rex Tillerson. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
Mexican Federal Police officers stand guard the Mexican side of the Mexico-US border in Tijuana, Baja California state, from where prototypes of the border wall, which US President Donald Trump will inspect on the outskirts of San Diego, in the US, are visible on March 13, 2018. Fresh off a cabinet reshuffle, President Donald Trump was headed for Democratic stronghold California on Tuesday to inspect prototypes of the controversial border wall with Mexico that was the centerpiece of his White House campaign. / AFP PHOTO / GUILLERMO ARIAS (Photo credit should read GUILLERMO ARIAS/AFP/Getty Images)
People hold signs reading "Trump, we will not pay for the wall" and "Trump, stop the mass deportations" near the border fence between Mexico and the U.S., in Tijuana, Mexico March 13, 2018. REUTERS/Edgard Garrido
Journalists gather at a rooftop near the US -Mexico border as President Trump is expected to inspect the border wall prototypes built outskirts San Diego, in Tijuana, Baja California State, Mexico, on March 13, 2018. Fresh off a cabinet reshuffle, President Donald Trump was headed for Democratic stronghold California on Tuesday to inspect prototypes of the controversial border wall with Mexico that was the centerpiece of his White House campaign. / AFP PHOTO / GUILLERMO ARIAS (Photo credit should read GUILLERMO ARIAS/AFP/Getty Images)
U.S. police officers use a ladder to climb up a truck parked in front of the prototypes of U.S. President Donald Trump's border wall, on the U.S. side of the current border fence, in Tijuana, Mexico March 13, 2018. REUTERS/Jorge Duenes
An agent faces Mexico while standing by the vehicle of U.S. President Donald Trump at the border near San Diego, California, where Trump reviewed wall prototypes designed to serve as a protective barrier against illegal immigrants, drugs and smuggled weapons, March 13, 2018. REUTERS/Kevin Lamarque
A U.S. Customs and Border Protection (CBP) Border Patrol Agent sits on horseback near U.S. President Donald Trump's motorcade during a tour of U.S.-Mexico border wall prototypes near the Otay Mesa Port of Entry in San Diego, California. U.S., March 13, 2018. REUTERS/Kevin Lamarque
TIJUANA, MEXICO - MARCH 13:Anti-Trump protestors demonstrate on the Mexico side of the border before the arrival of the U.S. President to inspect the prototypes of the proposed border wall on March 13, 2018 in Tijuana, Mexico. (Photo by Wally Skalij/Los Angeles Times via Getty Images)
The motorcade carrying US President Donald Trump drives past a US-Mexico border fence as Trump head for an inspection of border wall prototypes in San Diego, California on March 13, 2018. Donald Trump -- making his first trip to California as president -- warned there would be 'bedlam' without the controversial wall he wants to build on the border with Mexico, as he inspected several prototype barriers. The trip to the 'Golden State' -- the most populous in the country and a Democratic stronghold -- was largely upstaged by his own announcement that he had sacked Secretary of State Rex Tillerson. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
Mounted Border Patrol agents are seen as US President Donald Trump inspects border wall prototypes in San Diego, California on March 13, 2018. Donald Trump -- making his first trip to California as president -- warned there would be 'bedlam' without the controversial wall he wants to build on the border with Mexico, as he inspected several prototype barriers. The trip to the 'Golden State' -- the most populous in the country and a Democratic stronghold -- was largely upstaged by his own announcement that he had sacked Secretary of State Rex Tillerson. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
US President Donald Trump inspects border wall prototypes in San Diego, California on March 13, 2018. Donald Trump -- making his first trip to California as president -- warned there would be 'bedlam' without the controversial wall he wants to build on the border with Mexico, as he inspected several prototype barriers. The trip to the 'Golden State' -- the most populous in the country and a Democratic stronghold -- was largely upstaged by his own announcement that he had sacked Secretary of State Rex Tillerson. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
US President Donald Trump holds up a poster of before and after photos of a segment of the border wall prototypes with Chief Patrol Agent Rodney S. Scott (R) in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
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President Obama's Operation Phalanx and President George W. Bush's Operation Jump Start are the two most recent deployments of the National Guard at the border. The missions helped with immigrant apprehensions and drug seizures. But they were also costly and happened at a time when illegal border crossings were more frequent.

In 2006, Bush sent 6,000 National Guard troops to California, Arizona, New Mexico and Texas to support border patrol agents while new recruits trained. The deployment lasted two years and cost $1.2 billion. A 2012 Government Accountability Office report says the troops assisted in arresting about 186,000 undocumented immigrants and helped seize some 316,000 pounds of marijuana.

SEE MORE: Is Merit-based Immigration Our Solution? Let's Ask Canada

When first announcing the deployment, Bush said in a national address: "The United States is not going to militarize the southern border. Mexico is our neighbor and our friend. We will continue to work cooperatively to improve security on both sides of the border."

In 2010, two years after the end of Jump Start, Obama launched Operation Phalanx — a similar, though smaller mission. The 1,200 troops that were deployed assisted in the apprehension of over 17,000 undocumented immigrants and the seizure of some 56,000 pounds of marijuana. After a year, Obama mostly halted the ground mission to focus on aerial surveillance. The first year of the operation cost $110 million.

"We do hope that the deployment begins immediately. ... It will be strong. It will be as many as needed to fill the gaps that we have today," Nielsen said.

During Wednesday's press briefing, Nielsen said the National Guard would be deployed to "assist the Border Patrol." She also said Trump is still working with governors to hash out a plan. She didn't have many details about how many troops would go, how long they would stay and, specifically, what they would do.

Later in the day, Trump signed a proclamation to confirm the move. And Mexico's government said Nielsen informed them that the deployed U.S. troops at the border wouldn't be armed.

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