AUDIE CORNISH, HOST:
For the last few days, President Trump has touted plans to send troops to the U.S. border with Mexico. The details of that are now coming into focus. Here's Secretary of Homeland Security Kirstjen Nielsen yesterday at the White House.
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KIRSTJEN NIELSEN: The president has directed that the Department of Defense and the Department of Homeland Security work together with our governors to deploy the National Guard to our southwest border to assist the Border Patrol.
CORNISH: The details matter because the president can't just call up military forces on a whim to do domestic law enforcement. It's barred by a federal law dating back to the period after the Civil War, a law known as posse comitatus. Earlier today, George Mason University law professor Timothy MacArthur explained it to us.
TIMOTHY MACARTHUR: The law states that federalized military personnel cannot perform civil law enforcement functions. That means that anything short of search, seizure and arrest the military can assist with. And that would be providing surveillance, providing military equipment, training, information sharing, stuff of that nature.
CORNISH: So I understand this goes back to Reconstruction and the period right after the Civil War. What was going on why people decided to put this law into place?
MACARTHUR: Well, after the Civil War, during the Reconstruction era, what had happened is the southern states were either unwilling or unable to enforce civil laws. So at that time the federal government employed active duty military to keep the peace. And so what happened is Congress enacted the posse comitatus law. And that was due to the fact that the southern states didn't want military rule.
CORNISH: So ever since then, have presidents been able to get around this rule? I think you've written about the Civil Rights period where we saw federalized National Guard deployed.
MACARTHUR: Yeah. In 1957, President Eisenhower federalized the Arkansas National Guard to protect African-American students integrated into a previously all-white high school. By executive order, President George H.W. Bush deployed active duty troops to the city of Los Angeles in 1992 to quell the violence set off in response to the Rodney King verdict.
CORNISH: So what do we know about how the National Guard has been deployed in, say, support roles for border enforcement? I understand it's been done during the Bush and Obama administrations.
MACARTHUR: It has. In 2006, President Bush federalized the National Guard and sent them to the U.S.-Mexico border. In 2010, President Obama did the same. The real distinction here is what they're actually doing when they're mobilized and deployed to the border. As long as either a federalized National Guard or active duty military participate in passive activities, then it's allowed.
CORNISH: So the key word here is passive activities. What does that mean?
MACARTHUR: Well, that's generally a support role to the U.S. Border Patrol or anybody else that's securing our borders. It would be providing surveillance, providing assistance, providing military equipment and training on how to use that equipment. It's anything short of search, seizure and arrest.
CORNISH: So President Trump is working through governors to make this happen now. Is there a possibility that, say, a state could refuse - right? - could say look; we don't want to be involved in this?
MACARTHUR: Correct. The governor is the - you know, is the commander in chief for that - for those National Guard soldiers. And the governor here, she could say, you know what? We're not standing up our National Guardsmen and putting them on the border. So a governor can refuse the request of a president.
CORNISH: Is there a way that the president or administration could say, look; this is a national security issue?
MACARTHUR: Certainly the president can declare immigration as a national emergency and involuntarily mobilize National Guardsmen. But still the Posse Comitatus Act would still be in effect, which would not allow the National Guard to perform civil law enforcement functions.
CORNISH: What are you going to be looking for in the next few days as we hear more details about this?
MACARTHUR: Well, what I'm interested in hearing about is the actual application of how the president is going to put this into effect. Is he going to get the permission of the governors to bring them on in Title 10 federal status? What are the specifics of how you're going to do this and what you're going to actually do?
CORNISH: So it's not impossible. There are just limits.
MACARTHUR: Correct.
CORNISH: That's law professor Timothy MacArthur of George Mason University. Thank you for speaking with us.
MACARTHUR: Thank you for having me. Transcript provided by NPR, Copyright NPR.
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