Congress didn’t give President Donald Trump money for his border wall — and Mexico certainly isn’t paying for it — so now he’s asking his administration to go to legally questionable lengths to fund it anyway.

Trump has spoken with Secretary of Defense James Mattis about using Department of Defense money for the border wall, Pentagon spokesperson Dana White confirmed Thursday. The president reportedly noted that the Pentagon was getting so much money in Congress’s $1.3 trillion omnibus spending bill that it could surely afford the border wall, according to the Washington Post.

To be perfectly clear: This is not how government spending works.

Congress just passed a massive spending package — that Trump signed — boosting military and domestic funding on March 23. That spending bill 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.

And while the Pentagon received $700 billion as part of Congress’s government funding package, putting funding for the military at historic levels, that’s not a blank check to the Pentagon. That money is appropriated to specific programs — and spending it for the border wall instead could be illegal.

Despite this, Mattis seems to be humoring the president’s demands and coming up with some options to fund the border wall through the military. Most of these options will likely carry legal concerns and bear big political risks. And they will still fail to bring Trump close to the $25 billion funding level he wants for his wall.

Government spending, very quickly explained for Donald Trump

A quick civics primer: The legislative branch of government manages the nation’s purse strings, allocating money to federal agencies and authorizing how those funds can be used. It’s in Article I, Section 8 of the Constitution: Congress has the power ”to pay the Debts and provide for the common Defense and General Welfare of the United States.”

“Every line item is subject to tons of legislative, authorizing, and regulatory language with strict limitations on transfers and re-programmings,” Matthew Dennis, the Democratic spokesperson with the House Appropriations Committee, said.

And using federal funds for anything other than what Congress appropriates them for is illegal under that the Antideficiency Act, which was enacted in 1870.

In other words, Trump can’t simply call on the Department of Defense to use its excess money for his border wall. Even the Overseas Contingency Operations fund, the Pentagon’s emergency fund which can be used for “a future event or circumstance that is possible but cannot be predicted with certainty,” often derided as a “slush fund,” isn’t a blank check. Some amounts can be shifted, but like appropriations bill, all of that funding is also regulated and authorized by Congress.

It should be noted that going against Congress on government spending isn’t impossible — just extremely difficult, and carries the risk of legal and political backlash. So much so that an exclusive report in Just Security said few in Trump’s administration think using the Pentagon’s money for the border wall is a good idea — including Mattis. Nevertheless, the administration is exploring the idea.

If Trump is serious about this, he has some options. They’re questionable.

The Department of Defense has some room to move money around, but they all open Trump’s administration to legal challenges and political backlash and wouldn’t actually get Trump much closer to his $25 billion ask to fund the wall.

One option reportedly under consideration is using some of the counternarcotics funding, which sits at just over $500 million, Just Security’s Kate Brannen writes:

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 explicitly for counternarcotics operating programs, not construction. While it’s not explicitly prohibited to use the funding for construction, it’s likely Congress would be furious if it was spent this way.

Even though tapping into that fund would not need congressional approval, using only part of a $500 million fund wouldn’t get that 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.

The second option is using the emergency Military Construction, MILCON, funding. 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.”

As William C. Banks, a Syracuse University law professor pointed out in the New York Times last year, there are two other laws give Trump some wiggle room to move money around: the Economy Act of 1932, which could authorizes federal agencies to exchange supplies or services, and the Feed and Forage Act, a 1799 law which allowed the military to buy essential supplies like clothing or medicine. But according to the New York Times, “such an accounting trick has never been used to go around Congress on such a large scale, Mr. Banks said.”

In all, the options available to the Department of Defense and Trump are extremely limited in scope. And they all run the risk of going against Congress specifically for a project Republicans haven’t prioritized.

The spending bill largely ignored Trump’s immigration agenda, mostly because Republicans have an entirely different one

There’s no question that Congress’s spending bill does not reflect the hardline immigration agenda President Trump and his White House have long espoused.

As Vox’s Dara Lind writes, not only did Trump get very little of what he asked for, but “Congress is actually making an effort to rein in the Trump administration’s overspending on immigration detention instead of expanding it.”

Instead, in every spending fight since Trump took office, Republicans have repeatedly made clear that the wall and restrictionist immigration policy aren’t among their spending priorities.

This time the border wall did get some funding — $1.6 billion worth — which is much less than the $25 billion the White House asked for and came with a lot of strings attached; most of the funding will have to go toward repairing existing fencing or toward double fencing where barriers already exist.

On enforcement, Republicans, who went into 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. The bill also does not defund so-called “sanctuary cities,” something the White House specifically called on Congress to do.

Trump was so unhappy with the final spending package, he threatened to veto the bill altogether and shut down the government, but was walked off the ledge in the final hours.

Now, he seems to recognize that Congress won’t help him get the border wall and going around the legislative branch isn’t so easy.

His other option seems to be a GoFundMe campaign.