After floating the notion to several advisers last week, he told Speaker Paul Ryan, Republican of Wisconsin, that the military should pay for the wall in a meeting last Wednesday in the White House residence, according to three people familiar with the meeting.
Any repurposing of funds requires express congressional approval, which Trump is unlikely to get.
The $1.3 trillion spending bill, which Trump ruefully signed last week, only included $1.6 billion for fencing and levees on the border and just $641 million for new primary fencing in areas that do not now have barriers.
Trump campaigned on the promise that he would build the structure and somehow make Mexico foot the bill. Experts say those ideas would ultimately raise prices for US consumers, however, and would not result in the excess funding that would pay for the wall. In another meeting, Trump told his advisers that that the military could obviously afford paying for the wall because the latest budget included so much money for the Pentagon.
Meanwhile, the president suggested the Pentagon pay for the estimated $25 billion wall.
After failing to receive the money for his pet project from Mexico and the omnibus spending bill, Trump is said to be quietly petitioning, by way of the Pentagon, for taxpayer money. As a candidate, Trump repeatedly promised Mexico would fund the wall, but he has backed off those pledges in office, seeking federal money.
4 of 9 The president has warned there would be "bedlam" without the controversial wall he wants to build on the border with Mexico.
And he has tweeted that building "a great Border Wall" is "all about National Defence" and has called to "Build WALL through M!" - the military. "You could always increase them and take the revenue for that for resources", the same official said, who also described these options as Trump's "musings".
Sen. Minority Leader Chuck Schumer made Democrats' opposition abundantly clear: "First Mexico was supposed to pay for it, then US taxpayers, and now our men and women in uniform?"
During a September trip to Mexico, Mattis emphasized that US-Mexican military ties were strong and that the two countries shared common concerns. CNN reports that press secretary Sarah Huckabee Sanders declined to comment on possible military funding Tuesday.
The talks were a result of the $1.3 trillion spending package that the President begrudgingly signed last week that funded the government and averted a third government shutdown this year, but ultimately left Trump dissatisfied.