"We will continue to support DHS and other agencies as needed to keep our homeland secure", Mitchell said.
Some details: The money will be moved from procurement accounts to the drug interdiction accounts the Pentagon tapped a year ago for border projects.
Despite congressional opposition, Trump faced no consequences when making similar transfers a year ago, when the Pentagon canceled dozens of military construction projects to free up $3.6 billion and transferred $2.5 billion in counter-drug money.
Bob Salesses, the deputy assistant defense secretary, told reporters on Thursday that the funds are meant to help build 30-foot fencing on federally-controlled land in six border areas: San Diego and El Centro, California; Yuma and Tuscon, Arizona, and El Paso and Del Rio, Texas.
While no decisions have been made about additional funds being used from other military construction projects, Salesses confirmed that there are ongoing discussions about the possibility.
About 5,000 troops - including National Guard and active-duty forces - remain deployed to the USA southern border. "And the ability to do something about that effectively will make a huge impact on the security for this nation".
"Congress has the constitutional responsibility to determine how defense dollars are spent", Thornberry said in a statement.
"[This] undermines the principle of civilian control of the military and is in violation of the separation of powers within the Constitution", Rep. Mac Thornberry of Texas, the ranking Republican on the House Armed Services Committee, said in a prepared statement Thursday.
To take the funding, Trump used the counternarcotics law, as well as another little-known statute in US code, which allows the Pentagon, in the event of a national emergency requiring the deployment of troops, to divert military construction funds to pay for infrastructure needed by those forces. "The re-programming announced today is contrary to Congress's constitutional authority, and I believe that it requires Congress to take action".
The first part has the Pentagon transferring about $2.2 billion from 2020 appropriated funds under their general transfer authority, according to Pentagon documents.
Vermont Sen. Patrick Leahy, the top Democrat on the Senate Appropriations Committee called the wall a "vanity" project.
The contract, which The Washington Post notes is "meant to create a powerful, centralized computing system for USA military agencies", is estimated to be worth up to $10 billion over 10 years, and Microsoft won it last October after Amazon claims Trump used "improper pressure" and after a process Amazon says was full of "unmistakable bias". "Rather than investing in the procurement of critical platforms, this administration would rather dump another $3.8 billion into a wall on our southern border".
The project is expected to be completed in 12 to 24 months, with the U.S. Army Corps of Engineers overseeing the contracts for building the barrier, Salesses said.
Last month, the Pentagon received a request from within the Trump administration to build roughly 270 miles (435 km) of wall on the border, which would have cost about $5.5 billion.
But, CBP says only about one mile of new barriers have been built where none existed before.