President Donald Trump told reporters on Tuesday that his administration is “going to be guarding our border with our military,” reaffirming the White House’s intent to circumvent Congress’s spending bill and beef up border security.

After Congress’s $1.3 trillion spending bill largely ignored Trump’s immigration priorities, the president, who begrudgingly signed the funding package in late March, has called on his administration to find ways to fund the border wall anyway. Trump said he spoke with Defense Secretary James Mattis about the idea last week, but it’s still not clear what directives — if any — the president has given.

One idea, reportedly pushed by White House adviser Stephen Miller, is to send the National Guard down to the border, according to Just Security’s Kate Brannen. Trump certainly wouldn’t be the first president to call on the National Guard to help enforce the border (both Barack Obama and George W. Bush did the same). But it comes with legal constraints and could cost millions with little in returns, something Obama was criticized for.

There are a lot of legal limitations to using Pentagon money for the border across the board. While Congress’s government funding package allocated $700 billion for defense spending, it’s not a blank check to the Pentagon, and the money is appropriated to specific programs. Deviating from those regulations could be illegal.

Of course, there are some workarounds, but they have the potential to be very politically fraught — and likely pretty ineffective on the border. Trump, however, seems fixated on the idea.

Trump could use the military’s money on the border. His options are limited.

Currently, Congress has allocated $1.6 billion for the border wall through the Department of Homeland Security, with specific instructions that those funds had to go toward repairing existing fencing or toward double fencing where barriers already exist. In other words, Trump’s “big, beautiful” wall did not get funding.

On enforcement, Republicans, who went into the spending fight wanting more funding for the Department of Homeland Security to increase the number of beds for immigrant detainees and to expand the enforcement force, settled for more modest spending measures. The final compromise included funding for only an additional 328 Customs and Border Protection officers, and ICE will actually have to reduce the number of detention beds. Needless to say, this isn’t the kind of deportation force Trump’s administration was envisioning.

But Trump’s call to redirect money from the military toward the border is fraught. Of course, because it’s the legislative branch’s constitutional right to allocate government funding (and it’s illegal to use federal funds for anything other than what Congress appropriates it for), Trump is legally limited.

Like Obama and Bush, he could call on the National Guard to go down to the border. But if that directive comes from the federal government, the guards legally can’t act as law enforcement. The Posse Comitatus Act forbids using the military in civilian law enforcement. It leaves Trump two options: have states send down their guards, which means states would have to foot the bill, or have guards on the border in non-law enforcement roles.

For example in 2010, when Obama deployed 1,200 National Guard troops to the border, they fixed infrastructure, worked on surveillance, and processed arrests — similar to Bush’s 2006’s Operation Jump Start. But even this was very expensive and was largely criticized for being ineffective.

“Critics of the deployment include budget hawks, who say it is a waste of money, and residents here along the border, who say they are tired of seeing armed troops in their back yard,” the Washington Post reported at the time. “The 1,200 National Guard troops have helped Border Patrol agents apprehend 25,514 illegal immigrants at a cost of $160 million — or $6,271 for each person caught.”

Another option is to tap into the emergency military construction funding known as MILCON. Up to $50 million can be used without congressional approval in cases of national security or to protect US military officers. As Brannen writes, “using MILCON funding is also guaranteed to make members of Congress particularly angry, because it would be the military construction projects in their districts that would be directly losing money to the wall.”

Not to mention that $50 million won’t get Trump much mileage on the border. For context, Congress appropriated $445 million for 25 miles of levee fencing in the Rio Grande Valley and $251 million to replace 14 miles of existing secondary fencing in the San Diego sector.

Trump told reporters Tuesday that using the military on the border would be a “big step” toward achieving his border security agenda.

But reality paints a very different picture.