They couldn’t quite get him to understand it.

Just because Congress passed a spending bill doesn’t mean President Donald Trump could now treat the pile of taxpayers’ cash as a general fund to spend as he pleased on a huge and costly wall along the U.S.-Mexican border, aides explained to the President. Contrary to Trump’s experience as a real estate and marketing executive, this is not how the federal budget process works. The chatter on cable couldn’t trump Congress’ constitutional control over the budget.

But Trump wouldn’t hear it, according to two Administration officials. In Trump’s mind, Congress gave the money to the Pentagon, Defense Secretary Jim Mattis works for Trump, and Trump sees the wall as part of the United States’ defense. Ergo, he wants the Pentagon to build that wall. What was the point of being Commander-in-Chief — with a defense budget he boasts is “historic” — if Trump couldn’t make the military do his bidding?

White House aides told Trump that the budget he reluctantly endorsed wasn’t as flexible as budgets inside the Trump Organization. Congress explicitly forbade spending on a concrete barrier or in natural areas. Of the $1.6 billion allocated for border security — a far cry from the $25 billion Trump’s team sought as a starting point — most of it is for repairing existing fencing and conducting surveillance. When officials tried to explain to the President that the spending bill had no provisions for a military wall, Trump, in frustration, again blamed the filibuster for standing in his way, wrongly assuming fellow Republicans would go along with his desire to scrap it.

None of this got through, apparently, before Trump decamped last weekend for Florida. Trump sent a tweet on Saturday that read: “Build WALL through M!” (White House aides say the “M” in question was for Military, not Mexico.) “Military is rich again,” he tweeted.

The White House has tried to tamp down wall expectations — and head-off a rift with the Pentagon, which was already unhappy with the news Trump was bringing in a hardline national security advisor. “I can’t get into the specifics at this point,” press secretary Sarah Huckabee Sanders told reporters on Tuesday. “When we have an announcement on that, I’ll let you know.”

But when former Trump aide Jason Miller was on CNN a day later to make the case for having the Pentagon fund the wall, White House aides all knew what was coming back. “Don’t you think our border security is our national security?” Miller said.

The voices from cable news often overcome the policy insiders. With increasing frustration, the officials readied another presentation on why the federal budget wasn’t a piggybank for his whims. But after the briefing, Trump tweeted photos of fencing going up.

Trump has been stewing in recent days. The confluence of events — women speaking out about inappropriate ways they say Trump behaved, legal questions about payments made to these women and conservative backlash to his budget — left the President in a funk. Trump has told friends he feels let down by aides who didn’t do more on the wall, a major promise of his campaign.

Read More: It’s Now Season Two.’ How President Trump Turned White House Staffing Into a Cliffhanger

Inside the White House, those aides aggressively dismiss the criticism and defend their work. They note that Democrats offered $25 billion for wall funding in exchange for a fix on young people who came to the country illegally, including a pathway to citizenship—which the White House rejected. These White House aides also note there was little appetite for the wall among Republican leadership, including House Speaker Paul Ryan, who last week largely ignored Trump’s pleas for a military-funded wall during a meeting. This, too, Trump didn’t appear grasp. (The Washington Post first reported details of Trump’s meeting with Ryan.)

During his campaign Trump promised at every rally that he would build a wall and Mexico would foot the bill. Mexico has made plain that they won’t be picking up the tab, and the Republican-led Congress is no more willing to forego home-district projects to pay for a wall.

With members of Congress gone for a spring break, many officials expect Trump to tire of the military-funding idea by the time the Senate returns on April 9. “Has he ever stuck with anything in a sustained way that long?” one Republican official asked. “We’ll have 20 other headlines between now and then.”

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Write to Philip Elliott at philip.elliott@time.com.