President Donald J. Trump said the money to build the Mexican border wall is in the defense budget. The Pentagon's budget was part of a spending bill that became law Friday.

https://www.cnsnews.com/news/article/susan-jones/trump-suggests-using-military-funds-build-his-wall

"Building a great border wall", said President Trump, "is all about national defense". The President said national security is his most important responsibility.

https://www.bostonglobe.com/news/politics/2018/03/27/trump-privately-presses-for-military-pay-for-border-wall/lfttKj6Hr15VvmqOsiPP8K/story.html

https://www.washingtonpost.com/people/josh-dawsey/?utm_term=.3f9fea4df065

https://www.nbcwashington.com/news/politics/Trump-Suggests-Paying-for-US-Border-Wall-With-Pentagon-Funds-478141703.html

Senate Minority Leader Chuck Schumer said Friday's budget bill has no money for a new wall.

“I remind the President, he did campaign on the wall, and guess who he said would pay for it? How about going to Mexico and getting them to pay for it, because this budget doesn’t allow American citizens to pay for it,” said Schumer on Sunday.

https://nypost.com/2018/03/25/trump-suggests-the-military-might-build-the-border-wall/

The U.S. Supreme Court has never denied any President the ability to spend money on national defense.