This article has been updated with additional content.

When it comes to who’s going to foot the bill for his “big, beautiful” border wall, President Donald Trump is trying to change the M from “Mexico” to the “military,” asking the Pentagon to redirect a couple of billion dollars so that he can keep one of his central campaign pledges.

Few in Trump’s cabinet think using defense dollars to pay for the wall is a good idea, including Defense Secretary Jim Mattis, according to a Trump administration official who asked to remain anonymous because he was not authorized to speak publicly about internal discussions. But that doesn’t mean the idea is dead on arrival. Mattis feels that, at the very least, he owes Trump some options, but all of them are fraught with political and legal problems, the official said.

And none of them get the White House even close to the $25 billion Trump’s seeking. That’s because, perhaps to Trump’s surprise, Congress holds the power of the purse, meaning it gets to decide how taxpayers’ dollars are spent. And Congress did this just last week, when it passed a $1.3 trillion spending bill to fund the government through September. In that bill, which Trump reluctantly signed, lawmakers appropriated only $1.6 billion for a border wall. Meanwhile, Congress gave the Defense Department about $700 billion, which clearly got Trump thinking: Maybe some of that money can go toward the wall, as the Washington Post reported earlier this week. Since then, he has cryptically tweeted (and retweeted): “Build WALL through M!”

On Thursday, the Pentagon acknowledged that Mattis has had conversations with Trump on this topic, but would not provide any further details.

“What I can tell you is that the secretary has talked to the president about it, but I don’t have any specifics with respect to any more details,” Pentagon spokeswoman Dana White told reporters at a briefing.

The White House did not respond to a request for comment. When asked about it on Tuesday, White House Press Secretary Sarah Sanders said she couldn’t get into any specifics.

Trump is likely to be disappointed when he learns that thanks to the checks and balances built into the federal government, moving money around on a whim is not that easy. In fact, it’s usually illegal. The Pentagon has a little bit of wiggle room to move certain funds around without congressional approval, but generally, it must spend its money exactly the way in which Congress directs it.

This leaves only a few options for the Defense Department to redirect money legally to pay for Trump’s wall. It is now busy drafting those choices for the president, with Pentagon lawyers playing a pivotal role in the process.

According to the Trump administration official, the first option would be to tap into the Defense Department’s counternarcotics funding, which currently amounts to just over $500 million. In theory, the White House could make the argument that building a border wall with Mexico is for such purposes. However, the Pentagon’s funding is supposed to be for counternarcotics operating programs, and only allows for minor construction to facilitate counter-drug activities.

This means the Defense Department could technically move this money around without congressional approval but would risk burning a bridge with Capitol Hill, which could lead to lawmakers retaliating next year by revoking the authority and souring relations with Congress in the meantime. This would hurt the Defense Department, as well as the actual counternarcotics programs for which it is legitimately using the money. It is a legally viable route, but a politically perilous one.

The second option would require using money from the Defense Department’s emergency Military Construction — known as MILCON — funds. On Capitol Hill, MILCON funding is a big way for members of Congress to bring jobs and money to their district, so it is carefully protected and meticulously appropriated, separate and apart from all other defense spending.

Every military construction project must first be authorized by Congress. The only exceptions to this rule are the Defense Department’s two emergency MILCON authorities. The first authority allows up to $50 million to be spent on a construction project if it is for national security and to protect the safety of U.S. troops.

The second emergency authority has no spending cap, but it is only to be used in the event of a national emergency or a declaration of war. The last time the United States declared war was in World War Two, so that’s not going to happen.

Neither of these options get you close to the $25 billion that Trump wants for the wall, because the emergency MILCON funds can only be drawn from the annual MILCON budget, which is about $7-8 billion. The Defense Department is also explicitly prohibited from moving funds out of its regular spending accounts and putting it toward military construction. 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.

“All of the options to use DoD funds to pay for a border wall are a real stretch,” said Todd Harrison, a defense budge expert at the Center for Strategic and International Studies. “This is clearly not a Department of Defense activity and not the intent of the funding when Congress appropriated it. Using funding for purposes other than specified in the appropriations bill would likely generate lawsuits and bog this down in the courts.”

As for using an emergency authority, it “is also likely to cause this to end up in court because it is not immediately obvious that an emergency exists (what has suddenly changed?) and it is still not within the scope of what Congress intended when it provided this authority to DoD,” Harrison said.

Because all of these options would risk upsetting, if not enraging, Capitol Hill, the Pentagon is also exploring other ideas. For example, one proposal being floated is whether the Defense Department could deploy National Guard troops to the border with Mexico to fulfill Trump’s intent to have increased border security.

Here, new legal hurdles present themselves. Guard troops deployed under federal mobilization are prohibited from conducting law enforcement by the Posse Comitatus Act. However, National Guard troops mobilized by the states can conduct law enforcement but it must be in support of an emergency. Plus, states would get stuck with the bill. The Trump administration could also utilize Title 32 authority, where governors mobilize Guard troops, but the federal government foots the bill. It could be an attractive route for DHS and states, because DOD pays. However, because the Pentagon pays, DoD, along with the congressional defense committees, don’t like using Title 32 outside of true emergencies

It would not be the first time National Guard troops were sent to the Mexican border to increase security. President Barack Obama sent 1,200 National Guard troops to the Mexican border in 2010 to support law enforcement activities. They were there for a year to provide intelligence, surveillance and reconnaissance support; some intelligence analysis; and training, until Customs and Border Patrol could recruit additional officers to serve on the border. The move by Obama was viewed as politically savvy — he was facing a lot of pressure from Republicans on border security — but it was criticized as being inefficient and a waste of money. According to the Washington Post, “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.”

Today, Trump’s senior adviser Stephen Miller is a big fan of using National Guard troops on the border, according to the Trump administration official. It was also proposed the night before Trump’s March 25 tweet by conservative commentator Ann Coulter when she appeared on Jeanine Pirro’s show on Fox News.

“We engage in a lot of military actions around the world,” Coulter said. “I think it can be done right on our border as part of the defense. Have the Seabees do it.”

Pirro asked Coulter where Trump would get the money to do this.

“The same place Reagan took the money to invade Grenada,” Coulter said. “The same place he took the money to bomb Syria. He has money to spend on national defense, and this is a much bigger problem of national defense.”

But as Trump is now learning, it’s not that easy. Even though the Defense Department has a $700 billion budget, it’s up to Congress to decide how that money is spent, which it spells out every year in its defense appropriations and authorizations bills.

The bottom line: None of the options being considered by the Pentagon would get the White House anywhere close to the $25 billion that Trump wants for his wall. If he wants that much money, he has to ask Congress for it. Last week, it’s answer was: You can have $1.6 billion for your wall.

The question now becomes whether the White House would be satisfied with a much smaller amount from the Defense Department — maybe a few million dollars — and whether it is worth the backlash the Pentagon will likely get from Congress for using defense dollars on a project that few believe is necessary, let alone urgent.

Photo by Michael Reynolds-Pool/Getty Images