The Associated Press released a bombshell story Friday morning, revealing a draft Department of Homeland Security memo outlining a plan to call up 100,000 members of the National Guard to enforce the Trump administration's  crackdown on immigrants in 11 states.

The White House, through Press Secretary Sean Spicer, called the story false, but DHS admitted the memo itself was real, claiming it was an early draft of a plan that wasn't being seriously considered. Vox later obtained and published the full memo.

Serious or not, the memo raises the question: Does the federal government actually have the ability to call up the National Guard to enforce federal law? The answer depends on the law that's being enforced. 

The DHS memo was written to explore how to enforce President Trump's executive order increasing immigration enforcement. The memo is 11 pages long, and the National Guard component is just one idea it explores, along with hiring more Customs and Border Patrol (CBP) agents, building a border wall, and streamlining the process for investigating asylum claims.

One reading of the plan is that it was not calling for the Trump administration itself to mobilize the Guard, but to give state governors the option to do so, and to deputize Guard members to carry out CBP duties.

The National Guard has a number of functions, and can be called up using several federal laws. But only one, Title 10, can be ordered by the president, and only then in the case of a national emergency such as insurrection, rebellion, or invasion — with the consent of state governors. 

The Posse Comitatus Act, signed in 1878, forbids federal military personnel from enforcing domestic laws. However, Title 10 includes an exception to the Posse Comitatus Act that allows called-up forces to be used at the discretion of the president to support a request from a state governor. This is what's known as "federalizing" the Guard, and took place in the 1950s and 60s to enforce desegregation, and in 1992 after the Rodney King riots in Los Angeles.

The memo references mobilizing the National Guard under another federal statute, however: Title 32. This is full-time National Guard duty in the case of a statewide emergency, and has been implemented after disasters like Hurricanes Katrina and Rita, and after the September 11 attacks.

As the memo states"

"Pursuant to Title 32 of the United States Code, State National Guard components are employees of their respective states and are under the command of their Governors when they are not in federal service. Based on their training and experience, these men and women are particularly well-suited to assist in the enforcement of federal immigration law and augment border security operations by Department components." 

The memo lays out exactly how Guard members would be deputized to carry out the functions of the CBP, including the detention and processing of undocumented immigrants.

"I am directing the Commissioner of CBP and the Director of ICE to immediately engage with the Governors of the States adjacent to the land border with Mexico and to those states adjoining such border States [...] to authorize qualified members of the state National Guard, while such members are not in federal service [...] to perform the functions of an immigration officer in relation to the apprehension, investigation and detention of aliens in the United States."

However, the memo sidesteps whether federal troops carrying out these functions would be covered by exceptions to the Posse Comitatus Act, or whether it would require the writing of new statutes. National Guard troops have been deployed at the border before, most recently in 2010 and 2006 to assist with administrative and construction duties — but they had no authority to arrest or detain anyone.

The memo gives each governor the option of calling up their National Guard, and it's clear based on the politics of some of the states on the southern border (like California) that such call-ups wouldn't happen.

As of now, the memo appears to only be a draft of a plan that could be implemented, and not a blueprint for something about to happen. But if it did take place, it's likely that the opaque nature of the laws in question would ensure the attempt would be blocked in court, much like Trump's executive order banning travel from seven majority Muslim nations.