The Legal Restrictions Around President Trump Sending Troops To The U.S. Border

On Tuesday this week, President Trump surprised many when he announced he wanted to send troops to the US.. Border with Mexico.

"Until we have a wall and proper security we're going to be guarding our border with the military," said President Trump "That's a big step, we haven't really done that before, or certainly not very much before."

Well, Presidents Reagan, Clinton, George W. Bush and Barack Obama did send National Guard troops to the border in 1986, 1994, 2006 and 2010 respectively.

On Wednesday, Trump signed a proclamation directing National Guard troops to be deployed to the southern border, saying "lawlessness" at the border is "fundamentally incompatible with the safety, security, and sovereignty of the American people." And adding that his administration "has no choice but to act."

And on Thursday, the President told reporters he'd like to see between 2,000 and 4,000 troops deployed. There's still little detail, however, on exactly how many troops will be sent out, where they'd be sent, for how long, how much it would cost, and critically, whether they'd be armed.

All critical questions that require answers, because there's the matter of the law, an 1878 law to be specific, called the Posse Comitatus Act which generally limits the role of the military on U.S. soil.

Heather Cox Richardson, is a history professor at Boston College, and the author of "To Make Men Free: A History of the Republican Party." She tweets @HC_Richardson.