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!”