The troops have been used primarily for surveillance and other support activities. Troops are forbidden to conduct civilian police actions, such as detaining suspects and using force, under a law first signed by President Rutherford B. Hayes in 1878.
Trump's remarks on Tuesday appeared to catch many off guard including the military. Here are some of the questions raised by the president's comments.
1. How have presidents used troops in the past?
In 1986, Reagan declared drug smuggling a national security threat and launched a multipronged approach that included the National Guard as well as several law enforcement bodies. He also signed a bill that provided amnesty for 3 million migrants who had entered the U.S. illegally.
His successors, including George H.W. Bush and Bill Clinton, have all called in the troops.
George W. Bush deployed some 6,000 National Guard troops in 2006 for what proved to be a two-year mission, assisting the border guards with both undocumented immigrants and drug smuggling.
Barack Obama ordered a similar, if somewhat smaller, operation, in 2010. Around 1,200 National Guard troops were called up initially. By 2012, the numbers on the ground were scaled back and a greater emphasis was placed on aerial surveillance.
2. What is the current situation on the border?
After rising for decades, illegal immigration dropped after the 2008 recession that resulted in a sharp decline in economic opportunities.
In addition, the Obama administration deported more than 2 million undocumented migrants, more than any other president. The Obama administration focused primarily on convicted criminals and those apprehended shortly after coming across the border.
Illegal immigration took another downturn when Trump assumed office last year with his promise to crack down on illegal immigration and build a border wall.
3. So what prompted the president's decision to call on the National Guard?
While illegal immigration remains at a low level compared with in past decades, there has been an uptick in recent months, according to figures from Customs and Border Protection.
"The numbers we're releasing for March will indicate a staggering increase from last year," said a senior administration official during a conference call with reporters. "As of last April, we were at historic lows," the official said. "We are no longer at that point."
Critics say the president is acting more out of politics, noting that the $1.3 trillion spending bill he signed last month did not include the billions he wanted to build a border wall, a fundamental campaign pledge.
Trump has been speaking out and tweeting about immigration issues for the past several days, including a claim that "caravans" of migrants are crossing the border. (NPR has fact-checked some of those recent claims here.)
4. How much authority does the president have?
The role of the National Guard or any other troops is limited by the Posse Comitatus Act. President Hayes signed the measure in 1878 with the intent of ending the law enforcement role that federal troops were carrying out in the former Confederate states. The troops were supervising elections, chasing down criminal suspects and looking for illegal whiskey production.
The law, amended most recently in 1981, does permit the military to do things like help out on U.S. soil in the aftermath of a natural disaster. When it comes to the border with Mexico, this has meant providing surveillance and other support roles.