Although Speaker Paul Ryan and the White House initially claimed the omnibus spending bill funds Trump's wall, reality has set in and Trump is looking for new ways to fund construction.
Trump does not understand how the government or congressional appropriations work, however, so he's been telling his staff that the Pentagon should pay for it.
Moreover, Trump apparently believes the Pentagon owes him something because Congress appropriated so much money for defense.
Trump noted that the Department of Defense was getting so much money as part of a $1.3 trillion spending package that the Pentagon could surely afford the border wall, two White House officials said. The Pentagon received about $700 billion as part of the spending package, which Trump repeatedly lauded as “historic.”
“Build WALL through M!” Trump recently wrote on Twitter. Two advisers said that “M” stood for military.
From this I think we can also infer that Trump couldn't tell you why appropriating more money for the Pentagon is good for national defense. Most people would probably struggle to provide a good explanation, and it's debatable if the Pentagon really needs more money, but Trump doesn't even know what the money is for or how it's appropriated.
The Pentagon is not handed a blank check. With exceptions for top secret programs, virtually everything the Pentagon does is itemized by Congress and a border wall is not among those items.
Trump has grown frustrated watching constant TV criticism of the spending deal he signed last week and is determined to find a new way to fund the wall, several advisers said, privately grousing that his political supporters could become disenchanted without progress.