Can President Trump mobilize the California National Guard for border duty?

President Trump announced this week he wants to send American troops to the U.S.-Mexico border to prevent illegal crossings and drug smuggling.

At a press conference Wednesday, Homeland Security Secretary Kirstjen Nielsen said the nation’s current immigration system "rewards bad behavior" and "it's time to act."

So far the White House hasn’t said how many members of the National Guard will be deployed, or for how long, and how much the whole operation will cost.

When asked about the number of troops, Nielsen said, “it will be strong. It will be as many as needed to fill the gaps we have today...we do hope the deployment will begin immediately.”

Can President Trump do that? 

The National Guard serves a dual local and federal mission. Governors generally call upon the Guard to respond to natural disasters in their states. It happened earlier this year when California Army National Guard soldiers helped ferry mudslide survivors to safety in Montecito.

Guard units can also be mobilized by the President in case of federal emergency or to deploy overseas. In September, about 300 Cal Guard members headed to Jordan for a 9-month training and security mission. 

A post-Civil War-era law called the Posse Comitatus Act prohibits the use of military troops for domestic law enforcement. At the border, that means National Guard troops have been limited to supporting roles.

“They can’t go down to the border and start apprehending human traffickers and drug traffickers,” said Everard Meade, director of the Trans Border Institute at the University of San Diego. “What they can do is observe, they can provide operational intelligence.”

“What constitutes ‘domestic law enforcement’ and what amounts to simply supporting local authorities is a fine line,” said Kyndra Rotunda, professor of  Military & International Law at Chapman University. “Military troops can be used to respond to threats. But whether an active threat exists at the border is up for debate.”

Any threat posed by illegal crossings has dipped to historic lows, Meade said. “Undocumented immigration to the United States is at a 45-year low. Especially undocumented immigration across the land border. It’s a total red herring.” 

Rotunda said Governor Jerry Brown could object to military involvement at the border by claiming it violates the Posse Comitatus Act. 

“But whether a violation exists isn’t yet crystal clear,” Rotunda added. “It will ultimately depend how the forces are utilized.”

On Twitter Wednesday, Governor Kate Brown of Oregon said she would resist a request from the White House to mobilize her state's Guard units for border security. Only border states have been tapped for National Guard assistance so far.

If @realDonaldTrump href="https://twitter.com/realDonaldTrump?ref_src=twsrc%5Etfw">@realDonaldTrump asks me to deploy Oregon Guard troops to the Mexico border, I’ll say no. As Commander of Oregon’s Guard, I’m deeply troubled by Trump’s plan to militarize our border.

— Governor Kate Brown (@OregonGovBrown) April href="https://twitter.com/OregonGovBrown/status/981648079932633088?ref_src=twsrc%5Etfw">April 4, 2018 " class="embed-placeholder" data-cms-ai="0" ><blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">If <a href="https://twitter.com/realDonaldTrump?ref_src=twsrc%5Etfw">@realDonaldTrump</a> asks me to deploy Oregon Guard troops to the Mexico border, I’ll say no. As Commander of Oregon’s Guard, I’m deeply troubled by Trump’s plan to militarize our border.</p>&mdash; Governor Kate Brown (@OregonGovBrown) <a href="https://twitter.com/OregonGovBrown/status/981648079932633088?ref_src=twsrc%5Etfw">April 4, 2018</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

If such a showdown occurred in California, Trump would need congressional approval to authorize using military troops for routine border duty, Rotunda said.

It's happened before

Before Trump, other presidents have made the call for National Guard support at the border, and California has answered.

Presidents George W. Bush and Barack Obama both used the National Guard to supplement Customs and Border Patrol staffing. For those missions, troops did jobs like conducting surveillance, fence installation and maintenance, and training.

“This request  – as with others we’ve received from the Department of Homeland Security, including those for additional staffing in 2006 and 2010 – will be promptly reviewed to determine how best we can assist our federal partners,” said Lt. Col Tom Keegan, director of Public Affairs with Cal Guard.

 “We look forward to more detail, including funding, duration and end state.”

Currently on the border

The California National Guard is active right now on the Southwest border. 

55 members of Cal Guard’s Counterdrug program currently provide support to state and federal law enforcement counter-narcotics and counter-narcoterrorism efforts, the agency said. That includes missions like surveillance support at the San Diego Harbor, engineering to repair roads, fences and culverts, and criminal analysis.

There are 250 California National Guard members working in the Counterdrug program statewide.