In a series of tweets fired off Thursday morning, President Donald Trump delivered his most extreme threat to the governments of Mexico and Central America: "If they do not stop a caravan of Hondurans trying to reach the U.S. to apply for asylum, he vowed to use the U.S. military to "CLOSE THE SOUTHERN BORDER!"

It's unclear whether Trump is a full-fledged counterfeit of the 2,000-mile border with Mexico, or if it's using the threat simply to get America's southern neighbors to cooperate, or that it's just trying to rally its political base midterm elections.

But if sealing the border is realistically on the table, then that raises countless questions over the authority of the president to do so, the logistics of such an endeavor, and the widespread consequences it would have on Americans' ability to trade, travel and even eat.

"A shutdown of the border," said Peter Boogaard, a Homeland Security official in the Obama administration now working for FWD.us, an immigration advocacy group.

The first question, whether Trump can close the border, is a simple one to answer: yes.

"Said Gil Kerlikowske, US Customs and Border Protection's commissioner of the United States. "Logistically, that's possible." The gates are closed, and you say, "Right now we're not taking in."

Previous examples are rare. President George W. Bush partially closed the southern border following the 9-11 terrorist attacks, requiring full inspections of every incoming pedestrian. Ronald Reagan President of the United States of America, following the kidnapping and murder of a DEA agent in Mexico.

"(Reagan) wanted answers from Mexico and was not getting them, so he shut the border down," Thomas Homan, Trump's Head of Immigration and Customs Enforcement, told Fox News on Thursday. "It was not long before Mexico, unfortunately, found him, he'd been tortured and murdered, but they arrested the people who committed that crime. It worked. "

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The situation is more complicated if it is to be seen at a massive 2,000-mile stretch from Brownsville, Texas, to San Diego, Calif.

The National Guard can definitely be deployed in the United States. Several presidents have done so, mobilizing those to assist along the southern border, responding to natural disasters, and helping in the war on drugs. Trump has already done the same, issuing an order in April that feels 2,100 National Guard troops to help stop trump as a "crisis" level of illegal immigration.

But it's less clear that a president can order active-duty members of the Army, Navy, Air Force and Marines to patrol the southern border.

The Posse Comitatus Act of 1878 bars. Legal and military experts have long cited the law as a barrier to domestic deployments of the military.

"The Department of Defense really does have a role to play in this area of ​​the law," said Christine Wormuth, a former defense and defense director of the International Security and Defense Policy Center. Rand Corporation. "The United States public does not want the military to be policemen."

But the federal government has created some carve-outs that have allowed for domestic deployments. In 1991, the Congress passed a law that allows the Pentagon to assist the federal government and the state law enforcement of domestic anti-drug operations. That led to a 1997 incident in Texas, where a navy on a drug-monitoring mission shot and killed an 18-year-old who was on her family's ranch. U.S. law also allows the military to respond to insurgencies and the recovery of weapons of mass destruction.

The Trump administration has already been willing to push its borders, so it has been called "sanctuary cities" and end up that more than 1 million immigrants from deportation. Putting it all together and some military experts feel that they can not do it.

"He has to work with Congress, and there are some bureaucratic, legal procedures, but he can," said Frank Mora, a deputy assistant secretary for the Western Hemisphere at the Department of Defense in the Obama administration.

Mora made clear, however, that going through that process would be "ridiculous on so many levels." He said the idea of ​​deploying active-duty military along the border, but would be a disproportionate response to stop "women, children and young men" asylum.

"There are two hearings for this: his (political) basis, just three weeks before the election, and to intimidate our friends and allies into submitting and doing what the president wants," said Mora, now director of the director of the Kimberly Green Latin American and Caribbean Center at Florida International University.

Pentagon spokesman Lt. Col. Jamie Davis said the National Guard continues its deployment along the southern border, but that the Department of Defense "has not been tasked to provide additional support" as of Thursday afternoon.

Further complicating any closing of the border is the damage it would cause economically, not only in the four countries.

The U.S. State Department estimates that $ 1.7 billion in goods and services, and hundreds of thousands of people, legally cross the border each day. The U.S. gets nearly half (44 percent) of its fresh fruits and vegetables from Mexico, according to the U.S. Department of Agriculture.

"There is a reason for entry into existence," said Mark Krikorian, executive director of the Center for Immigration Studies, a group that advocates for lower levels of legal and illegal immigration.

That's why Krikorian, who wants the migrant caravan stopped, says Trump can not seriously consider the closure of the border. Krikorian believes Trump is just bluffing to get Mexico to stop the caravan for him, or simply using the threat to rile up the Republican base before the midterm elections.

This Trump articulates what a lot of ordinary people feel when they see, "a caravan of migrants headed toward the U.S., he said. "That's part of his strength – he gives you a voice that is normal."

more: Mexican government sends federal police to intercept caravan of U.S.-bound migrants

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